Investigate linker warnings

https://github.com/rust-lang/rust/pull/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:

• https://github.com/rust-lang/rust/blob/4b4007a500e27f708038e81549d7ad6437ce1b71/src/bootstrap/src/core/build_steps/test.rs#L1869-L1872
• https://github.com/rust-lang/rust/blob/4b4007a500e27f708038e81549d7ad6437ce1b71/src/bootstrap/src/core/build_steps/test.rs#L3587-L3588
• https://github.com/rust-lang/rust/blob/4b4007a500e27f708038e81549d7ad6437ce1b71/src/bootstrap/src/core/builder/cargo.rs#L276

@rustbot label S-tracking-issue

Start by reproducing the listed linker warnings in the affected tests and builds, then inspect the warning suppressions in src/bootstrap/src/core/build_steps/test.rs and src/bootstrap/src/core/builder/cargo.rs at the referenced lines. Compare the linked pull request and comments to identify each warning's cause; done means the warnings are addressed rather than silenced.