[dlfcn] E: Add dlfcn-weak-c tests for STB_WEAK loader semantics

[esaurez]

Summary

Adds a new src/dlfcn-weak-c/ regression suite covering STB_WEAK
undefined-symbol semantics through the public POSIX dlfcn interface
(dlopen/dlsym). The suite is the canonical end-to-end test for the
in-process Nanvix loader change that adds STB_WEAK handling to
get_symbol_value() — once that change lands upstream, this suite
becomes the test that pins the behaviour in place.

Why this matters

The System V gABI (chapter "Symbol Table") specifies that an unresolved
STB_WEAK undefined symbol takes value zero at dynamic link time, with
the program responsible for null-checking before use. Every mainstream
ELF loader (glibc elf/dl-lookup.c, musl ldso/dynlink.c, FreeBSD
rtld-elf/rtld.c, Android Bionic linker/linker_relocate.cpp)
implements this rule. The Nanvix loader work mirrors that contract.

We need a hermetic C-level test driven through the public POSIX dlfcn
API — independent of any specific consumer (cpython, Rust startup
crates, libposix) — so that future loader refactors cannot silently
re-break the contract.

Test matrix

7 cases, all in main.c. Magic string "ok" is written on full
success; assert() aborts on failure (matches the sibling dlfcn-c,
dlfcn-pie-c, dlfcn-global-c, dlfcn-needed-c convention).

#	Fixture	Relocation	Symbol	Expected behaviour
5	libstrong-missing.so	R_386_JUMP_SLOT	strong_missing	dlopen(RTLD_NOW) fails
1	libweak-func-resolved.so	R_386_GLOB_DAT	main_callback	resolves to main exe; try_callback()==70
2	libweak-func-missing.so	R_386_GLOB_DAT	missing_callback	&fn == NULL; try_callback() == -1
3	libweak-data-resolved.so	R_386_GLOB_DAT	weak_data	resolves to main exe; read_weak_data()==99
4	libweak-data-missing.so	R_386_GLOB_DAT	missing_weak_data	&data == NULL; read_weak_data() == -1
6	libweak-plt-resolved.so	R_386_JUMP_SLOT	main_callback	resolves to main exe; PLT call returns 70
7	libweak-plt-missing.so	R_386_JUMP_SLOT	missing_plt_callback	dlopen(RTLD_NOW) succeeds

Case 5 runs first so that a regression in the strong-undefined
path is reported independently of the weak cases. It is also the only
case that passes against a loader without STB_WEAK support — see the
validation section below.

Case 7 deliberately does not call the function pointer it resolves.
The JMP_SLOT was zeroed by the loader (weak undefined missing → 0);
invoking it would jump to NULL. The case only validates that
dlopen(..., RTLD_NOW) accepts the .so despite the weak undefined
PLT entry.

Relocation-class coverage

The weak NULL-guarded pattern if (&fn) fn(); (in libs/weak_func.c
and libs/weak_data.c) forces a GOT-indirect call/load and only
exercises R_386_GLOB_DAT. To also exercise weak references via the
PLT, this PR adds libs/weak_func_plt.c (an unguarded call) which
emits R_386_JUMP_SLOT. Together, the two helpers cover both
relocation classes the loader must handle for weak undefined symbols
in i686 ELF.

The strong-undefined regression guard (libs/strong_missing.c) is
unguarded by construction and emits R_386_JUMP_SLOT.

Build-time fixture verification

The Makefile asserts via readelf -rW and readelf -WsD that each
produced .so contains both:

1. The intended relocation type AND symbol name (proves the fixture is
   exercising the symbol we think it is, not some accidental CRT-injected
   weak symbol).
2. The intended dynamic-symbol binding (WEAK / GLOBAL) and that the
   symbol is UND.

Helper .so files are compiled with the suite-wide CFLAGS, keeping
the test representative of the project's normal -O2 + warning
environment.

Linker flags

Main executable: -pie -rdynamic -Wl,--no-dynamic-linker

• -pie + -rdynamic: export main_callback and weak_data in the
  main exe's .dynsym so the resolved cases can look them up
  (matches the existing dlfcn-pie-c pattern).
• --no-dynamic-linker: Nanvix uses an in-process loader via syscalls,
  not a separate ld.so.

Validation

• End-to-end with the STB_WEAK loader change applied: all 7 cases
  pass against a Nanvix microvm guest. The full test runs in standalone
  mode via ./z test.
• Case 5 alone against a loader WITHOUT the STB_WEAK change: also
  passes (sanity-validated by temporarily commenting out cases 1-4 and
  6-7 and rebuilding). This confirms the regression-guard semantics
  (dlopen(RTLD_NOW) must continue to fail on missing strong UND
  symbols regardless of the weak-symbol code path).

Merge order

This PR can land any time. Cases 1-4 and 6-7 will only pass once the
Nanvix loader STB_WEAK change is merged. If this PR lands first, the
posix-tests CI run on a Nanvix build without the loader change would
fail cases 1-4/6-7 — that's the intended signal once the matching
nanvix-side PR opens. The cleanest sequencing is to merge this PR
roughly the same release cycle as the loader change.

[esaurez]

Superseded by upstream PR nanvix/posix-tests#176, which carries the same test contribution -- rebased onto upstream main, scope-trimmed to only the test code (the fork PR also carried unrelated local-env drift: a CI workflow downgrade from v2.2.0 to v2.0.2, a nanvix-version downgrade from 0.16.17 to 0.15.26, z.ps1/z.sh changes, .gitignore tweaks, and z.py imports cleanup -- all dropped), deep-reviewed (one SHOULD-FIX applied: removed fork-specific BINARIES_DIR/LIBRARIES_DIR/READELF ?= fallbacks and Windows REMOVE macro from the suite Makefile to match siblings; three NITs applied: stripped non-ASCII em-dashes and right-arrow, dropped 'static' qualifier on test helpers to match dlfcn-c convention, added per-case R_386_GLOB_DAT / R_386_JUMP_SLOT relocation comments). Full posix-tests integration suite passes against the 0.16.17 release nanvix. Closing this fork PR; tracking continues upstream.