[build] F: Drop -Wl,--allow-multiple-definition (no longer needed)

[esaurez]

Important

DO NOT MERGE until ALL of the following have landed and propagated:

1. nanvix/nanvix#2487 — sys-ffi crate split that removes 36 of 37 duplicate strong symbols.
2. nanvix/newlib#17 — weakens _do_start in newlib's crt0.S so libnvx_crt0's strong override wins (the 37th duplicate).
3. nanvix/toolchain-gcc#14 — pins the new newlib commit and republishes the toolchain image (DRAFT until [ci] E: Update nanvix workflow refs to v1.15.0 #17 merges).
4. The Docker image at ghcr.io/nanvix/toolchain-gcc has actually been republished from [ci] E: Pin nanvix to v0.12.538 #14 with the new newlib bundled in.
5. The parent E: PR that this PR is stacked on top of (the one that added the libnvx_crt0.a wildcard link entry with the safety flag).

If merged early, the link step for the test ELF will fail with multiple definition of __kcall_* errors against the unpatched toolchain image.

Summary

Drops -Wl,--allow-multiple-definition from the test ELF link line. The flag was added by the parent E: PR as a temporary safety guard while 37 duplicate strong symbols were still present in both libnvx_crt0.a and libposix.a. Once those are resolved structurally, the flag is no longer needed and the SSE-aligned _do_start from libnvx_crt0.a finally wins at the entry point.

Why the flag was needed

libnvx_crt0.a and libposix.a both contained byte-identical duplicate strong definitions of:

• 34 sys-crate __kcall_* FFI wrappers
• _do_exit_thread
• _do_start_thread
• _do_start (which also collides with newlib's prebuilt crt0.o)

-Wl,--allow-multiple-definition told ld to silently take whichever copy it saw first. Since the copies were byte-identical (same Rust source, same --release profile, same compiler), the outcome was semantically equivalent to deduplication -- but the flag also hides legitimate symbol collisions, which is why it was always intended to be temporary.

How the duplicates have been resolved

• 36 of 37 -- nanvix/nanvix#2487 introduces a sys-ffi crate that owns all #[no_mangle] re-exports of sys::kcall::*. Only libposix.a links sys-ffi, so libnvx_crt0.a no longer duplicates the __kcall_* / _do_exit_thread / _do_start_thread symbols.
• The 37th (_do_start colliding between libnvx_crt0.a and newlib's prebuilt crt0.o) -- nanvix/newlib#17 declares newlib's stub .weak so libnvx_crt0's strong SSE-aligned _do_start wins cleanly without the flag.
• Toolchain image republish -- nanvix/toolchain-gcc#14 bumps the pinned newlib commit and triggers a republish of ghcr.io/nanvix/toolchain-gcc so all consumers get the new newlib.

Until ALL three changes land AND the toolchain image is republished, this PR's link step will fail. That is why this is a deliberately-separate follow-up PR rather than being squashed into the parent E: PR -- it gives reviewers the option to merge the E: compat fix immediately (preserving backward compatibility with both pre-cutover and post-cutover sysroots, with the safety flag) and circle back to drop the flag in a single small PR once the toolchain catches up.

Side benefit: SSE-aligned _do_start at the entry point

With the flag in place, ld silently takes newlib's 9-byte unaligned _do_start (crt files come first in the gcc spec). Dropping the flag lets libnvx_crt0's 18-byte SSE-aligned variant -- which aligns %esp to a 16-byte boundary before the CALL instruction, as required by the i386 SysV ABI for SSE-using callees (movaps / movdqa) -- finally win. This fixes a latent SSE-alignment bug present in shipping binaries today.

Validation

Built end-to-end against a locally-rebuilt toolchain image carrying the nanvix/newlib#17 change. Test ELF links cleanly without --allow-multiple-definition. Disassembling the resulting binary confirms _do_start at the entry point is the SSE-aligned variant:

<_do_start>:
    and    $0xfffffff0, %esp      ; 16-byte align (was missing)
    mov    %esp, %ebp
    sub    $0x8, %esp              ; alignment padding (was missing)
    push   %ecx
    push   %edx
    call   _start

[esaurez]

Folded into the simplified main PR #74. The original two-PR design (add the flag in E:, drop it in F:) assumed a transitional period where the link line needed to work both before and after the toolchain image was republished. We're now landing this whole work only after Wave 1 (nanvix/nanvix#2487) + Wave 2 (nanvix/newlib#17 + nanvix/toolchain-gcc#14) have merged and the toolchain image has been republished, so the safety flag was never needed in the first place. The main PR has been rewritten as a single straightforward change that adds libnvx_crt0.a to the test ELF link line without --allow-multiple-definition.