[libc] [llvm] [libc] Provide `LIBC_TYPES_HAS_INT128` (PR #84149)

[Guillaume Chatelet via llvm-commits]

gchatelet wrote:

@lntue should we also provide aliases (`using int128_t = __int128_t`) as we do for `_Float16` et al.?
It looks to me like it would be more consistent.

https://github.com/llvm/llvm-project/pull/84149