When running Emscripten tests, rustbuild should tell compilertest the version of the Emscripten LLVM, not the host LLVM

[kennytm]

Discovered in #48983 (comment). It seems rustbuild will always passes information about the host LLVM config, even when running Emscripten tests.

rust/src/bootstrap/test.rs

Lines 911 to 920 in 3926453

	if build.config.llvm_enabled {
	let llvm_config = builder.ensure(native::Llvm {
	target: build.config.build,
	emscripten: false,
	});
	let llvm_version = output(Command::new(&llvm_config).arg("--version"));
	cmd.arg("--llvm-version").arg(llvm_version);
	if !build.is_rust_llvm(target) {
	cmd.arg("--system-llvm");
	}

This makes min-llvm-version reports the wrong value and failed to ignore some tests.

cc #48757 (probably a regression introduced by this PR)

[kennytm]

I think changing that false to target.contains("emscripten") is enough?