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

[jyn514]

@rustbot label L-linker_messages

[jyn514]

cc #136096

[madsmtm]

For the ignoring duplicate libraries warning, I see three ways of resolving it:

1. Pass the -no_warn_duplicate_libraries flag to the linker. I'm unsure when this was introduced, but it isn't available in Xcode 9.2 on macOS 10.12.
2. Deduplicate libraries in rustc before passing them onwards to the linker. Might be difficult because of various ordering requirements?
3. Filter it out from the stderr manually by matching on strings.

[jyn514]

i do not think we should do 2, i think that will come back to bite us hard. 1 would be my preference, but i don't want to change the mechanism between old and new deployment targets. so that really just leaves 3 unless we're willing to say 10.12 isn't supported / always warns.