bootstrap: correct reading of flags for llvm

[jonhoo]

First, this reverts the CFLAGS/CXXFLAGS of #93918. Those flags are
already read by cc and populated into Build earlier on in the
process. We shouldn't be overriding that based on CFLAGS, since cc
also respects overrides like CFLAGS_{TARGET} and HOST_CFLAGS, which
we want to take into account.

Second, this adds the same capability to specify target-specific
versions of LDFLAGS as we have through cc for the C* flags:
https://github.com/alexcrichton/cc-rs#external-configuration-via-environment-variables

[rust-highfive]

r? @Mark-Simulacrum

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[jonhoo]

Hold on before merging this — I just realized there's actually another bug in the pre-existing logic that is what caused me to submit the (partially erroneous) fix is #93918. Specifically, we set both CFLAGS and CXXFLAGS based on builder.cflags:

rust/src/bootstrap/native.rs

Line 553 in 8d6f527

let mut cxxflags: OsString = builder.cflags(target, GitRepo::Llvm).join(" ").into();

This is wrong — when computing the base arguments for CXXFLAGS, we need to use builder.cxx not builder.cc, which means this bit needs to be changed to take into account whether it's trying to grab CFLAGS or CXXFLAGS:

rust/src/bootstrap/lib.rs

Lines 944 to 952 in 8d6f527

	fn cflags(&self, target: TargetSelection, which: GitRepo) -> Vec<String> {
	// Filter out -O and /O (the optimization flags) that we picked up from
	// cc-rs because the build scripts will determine that for themselves.
	let mut base = self.cc[&target]
	.args()
	.iter()
	.map(|s| s.to_string_lossy().into_owned())
	.filter(|s| !s.starts_with("-O") && !s.starts_with("/O"))
	.collect::<Vec<String>>();

[jonhoo]

I've updated the PR to handle CXXFLAGS separately from CFLAGS throughout src/bootstrap. This also required teaching compiletest about --cxxflags.

[Mark-Simulacrum]

r=me with squashed changes

[jonhoo]

squashed and rebased onto master

[Mark-Simulacrum]

@bors r+

[bors]

📌 Commit 9c05f0b has been approved by Mark-Simulacrum

[Mark-Simulacrum]

See #94719 (comment) -- approving for beta backport.