aarch64-linux: link libgcc for outline atomic helpers

[dcalavrezo-qorix]

rust_binary targets built with the ferrocene_aarch64_unknown_linux_gnu toolchain fail to link when they pull in C/C++ code that performs atomic operations (e.g. std::atomic, std::shared_ptr refcounts, mutexes via C++ interop):

    undefined reference to `__aarch64_ldadd4_acq_rel'
    undefined reference to `__aarch64_cas2_acq_rel'
    undefined reference to `__aarch64_swp4_acq_rel'

Root cause: GCC 10+ defaults to "outline atomics" on aarch64. Rather than emitting LSE (ARMv8.1) or LL/SC atomics inline, the compiler emits calls to helper functions in libgcc that select an LSE or a backward-compatible path at runtime. Those helpers are defined only in the static libgcc.a; the shared libgcc_s.so does not export them. rustc links with -nodefaultlibs and adds -lgcc_s but not -lgcc, so the helpers are never offered to the linker.

Fix: append -Clink-arg=-lgcc to the aarch64-unknown-linux-gnu toolchain's extra_rustc_flags so libgcc.a is on the link line. It is placed last because the linker resolves a static archive only against symbols already referenced to its left, so libgcc.a must follow the C/C++ objects and libraries (-lstdc++ etc.) that reference the helpers.

This resolves the outline-atomics half of the aarch64 GCC link failure without requiring downstream repositories to inject the flag via .bazelrc.

MODULE.bazel.lock is regenerated (bazel mod deps --lockfile_mode=update) to reflect the added flag.

Refs: #27, eclipse-score/communication#328

[AlexanderLanin]

AI-assisted review (Claude):

This line is now the only difference between the x86_64 and aarch64 linux flag lists, and its placement is load-bearing. Since CI only validates lockfile consistency and the repo has no build/link test, a future maintainer aligning the toolchains or reordering flags could silently drop it and reintroduce the __aarch64_*_acq_rel link failure with no CI signal. A short comment makes the intent and the ordering constraint durable:

Suggested change

	"-Clink-arg=-lc",
	"-Clink-arg=-lgcc",
	"-Clink-arg=-lc",
	# aarch64 GCC uses outline atomics; the __aarch64_*_acq_rel helpers live
	# only in static libgcc.a. Must stay last so it follows the C/C++ objects
	# that reference them (static archives resolve left-to-right).
	"-Clink-arg=-lgcc",