[libc] [llvm] [libc] Remove obsolete LIBC_HAS_BUILTIN macro (PR #86554)

[Nick Desaulniers via llvm-commits]

nickdesaulniers wrote:

> We could potentially stipulate that people need to use newer `GCC`, but it's a little annoying when this worked around it previously.

ACK'ing this point. (I don't want it to seem that I'm ignoring it)

Such a policy would effectively replace [our stated compiler support window](https://libc.llvm.org/compiler_support.html#minimum-supported-versions) with [LLVM's much larger compiler support window](https://llvm.org/docs/GettingStarted.html#host-c-toolchain-both-compiler-and-standard-library).  I would feel more comfortable pursuing this expanding this window with additional build bot coverage for specific compiler versions.

There's certainly a chick-and-egg problem of having a too modern of a compiler support window preventing you from growing your userbase.  Though unless we have significant known users stuck with older toolchains, my slight preference for now is to be more aggressive with requiring newer toolchains.

I think the additions to the rpc server mitigate the need to make a decision here presently.  Do consider requesting changes to signal N'ACK'ing this patch if you strongly about it @jhuber6 .  I really don't feel strongly either way (despite the exposition dump lol).

I'll wait for presubmit results and @gchatelet 's feedback when he's comes back online, but LGTM.

https://github.com/llvm/llvm-project/pull/86554