x86_64 self-hosted backend universal-libc tests are failing

[andrewrk]

Zig Version: 0.11.0-dev.2634+b0186f310

zig/test/tests.zig

Lines 962 to 964 in b0186f3

	// TODO get universal-libc tests passing for self-hosted backends.
	if (test_target.use_llvm == false and mem.eql(u8, options.name, "universal-libc"))
	continue;

Until this issue is resolved, the CI does not provide test coverage for the x86_64 self-hosted backend passing the universal-libc tests.

Related: #15329

This is a prerequisite to making the x86_64 self-hosted backend the default backend for debug builds.