[nvx] E: Cut over to nvx-crt0 startup crate

[esaurez]

[nvx] E: Cut over to nvx-crt0 startup crate

Completes PR-11 by moving the executable startup code (_do_start / _start / c_trampoline / ARGC/ARGV / environ / __nanvix_env_init invocation / argp/envp parsing) out of nvx exclusively into the nvx-crt0 crate that was added in PR-11a (#24).

After this commit:

• libposix.a no longer contains _start / _do_start / main UND, so it is safe to embed in .so shared objects.
• libnvx_crt0.a becomes the sole provider of those symbols and must be linked (under --whole-archive) into every guest executable.
• The ad-hoc objcopy --weaken-symbol=main post-link hook (the POST_STATICLIB_HOOK_posix macro in build/make/generic-guest-staticlibs.mk) is no longer required and is removed.

What changed:

• src/libs/nvx trimmed to pub mod pie + pub fn init() / pub fn cleanup() + panic handler. Removed default staticlib / standalone features that previously gated the startup code. Module-level docs point readers to nvx-crt0.
• src/libs/posix: dropped staticlib = ["nvx/staticlib"] feature and the corresponding staticlib default. pwd / utime / sys::ioctl #[cfg(all(syscall, staticlib))] simplified to #[cfg(syscall)] (same effective behaviour: syscall was always on whenever staticlib was).
• 29 Rust no_std guest binaries (benchmarks / daemons / tests / hello) now declare nvx-crt0 = { workspace = true, features = ["rust-main"] } plus the matching log-level feature forwarding (trace / debug / info / warn / error / panic). Each main.rs adds extern crate nvx_crt0; so the static lib is force-loaded.
• 5 binaries that read the kernel argv blob directly migrated from nvx::ARGC / nvx::ARGV to nvx_crt0::ARGC / nvx_crt0::ARGV.
• 3 binaries that pulled nvx/standalone (mount-bench / mount-test / mount-multipart-test) dropped that feature — standalone-mode gating now lives entirely in the application crate.
• build/make/generic-guest-staticlibs.mk: dropped staticlib from the default GUEST_STATICLIB_FEATURES and deleted POST_STATICLIB_HOOK_posix plus both call sites.

Validated end-to-end:

• hello-c (single ELF, multi-process mode) prints expected output.
• hello-cpp (libstdc++ static ctors + _init / _fini) works.
• envtest (argv / environ / getenv) works.
• hello-c in standalone+ramfs mode works.
• CPython hello + lxml + HTTP-server smoke tests PASS in standalone mode (483 ms hello execution) when paired with PR-11b-cpython ([nanvix] E: Link libnvx_crt0.a into python.elf cpython#2) which adds $(LIBNVX_CRT0) to python.elf's LIBS. See dependency note below.

IMPORTANT downstream dependency:

After this commit, any C executable that previously relied on libposix.a's _start MUST be relinked against libnvx_crt0.a under --whole-archive. Without it, the link silently succeeds (no undefined-reference error) because libc.a ships a WEAK fallback _start from newlib — but the resulting binary hangs at runtime, as the newlib default _start does not understand Nanvix's process-spawn trap-frame layout. The matching CPython change is PR-11b-cpython (esaurez/cpython#2, stacked on esaurez/cpython#1 / PR-10).

Stacked on PR-11a (#24).

Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com