Investigate linker warnings

[jyn514]

#119286 unsilences linker warnings by default, and then promptly silences them again for rust-lang/rust itself. We should investigate and fix them.

So far I have seen the following kinds of warnings:

• ignoring duplicate libraries: -lm on MacOS when running any tests/ui-fulldeps test, and also when building rustc itself for stage 2.
• ld: cannot export hidden symbol compiler_builtins::int::leading_zeros::__clzdi2::ha6dd05e8d8124fab when building compiler-builtins
• ld: cannot export hidden symbol ___llvm_profile_filename when building tests/ui/issues/issue-85461.rs
• ld: build/x86_64-unknown-linux-gnu/stage1-tools/cg_gcc/mini_core.mini_core.2ab949a2a8595fff-cgu.0.rcgu.o: relocation in read-only section `.rodata.global.c' and ld: creating DT_TEXTREL in a shared object when building rustc_codegen_gcc

and they are silenced in these places:

• rust/src/bootstrap/src/core/build_steps/test.rs

  Lines 1869 to 1872 in 4b4007a

  	// FIXME: on macOS, we get linker warnings about duplicate `-lm` flags. We should investigate why this happens.
  	if suite == "ui-fulldeps" && target.ends_with("darwin") {
  	hostflags.push("-Alinker_messages".into());
  	targetflags.push("-Alinker_messages".into());

• rust/src/bootstrap/src/core/build_steps/test.rs

  Lines 3587 to 3588 in 4b4007a

  	// cg_gcc's build system ignores RUSTFLAGS. pass some flags through CG_RUSTFLAGS instead.
  	.env("CG_RUSTFLAGS", "-Alinker-messages")

• rust/src/bootstrap/src/core/builder/cargo.rs

  Line 276 in 4b4007a

  self.rustflags.arg("-Alinker-messages");

@rustbot label S-tracking-issue