[libc] [llvm] [libc][math] Implement single precision lgamma function (PR #205490)

[via llvm-commits]

zimmermann6 wrote:

I got confirmation from the CFG group that signgam is a POSIX issue. Thus if you don't support POSIX but only the C standard, you don't have to worry about it.

https://github.com/llvm/llvm-project/pull/205490