[dynlink] Toolchain dynamic-executable mode (clang driver)

[ppenna]

Part of startup dynamic-linking epic (Path A: crt0 self-linker).

Goal

Let clang produce a dynamic nanvix executable that records DT_NEEDED for libc.so/libm.so, instead of static-only.

Scope (llvm-project Nanvix driver)

• Add a dynamic mode: drop --no-dynamic-linker, emit -pie --export-dynamic, keep --hash-style=sysv.
• Make -lc/-lm resolve to libc.so/libm.so (DT_NEEDED) rather than forcing -Bstatic.
• Keep static the default; gate dynamic behind a flag/triple.

Acceptance

• clang --target=i686-unknown-nanvix (dynamic) yields ET_DYN with .dynamic + DT_NEEDED=libc.so,libm.so and .hash.
• Static path unchanged. lit driver test for dynamic flags.