[libc] [llvm] [reland][libc][NFC] Use user defined literals to build 128 and 256 bit constants (PR #81835)

[Guillaume Chatelet via llvm-commits]

gchatelet wrote:

Looking at Fuschia BuiltBot failures I also found quite a few other issues
https://lab.llvm.org/buildbot/#/builders/98/builds/33387

The fixes appear as individual commits in this patch, PTAL @lntue 

https://github.com/llvm/llvm-project/pull/81835