Also enable half-precision variants of tgamma, which were previously
missing.
Note that unlike recent work, these builtins are not vectorized as part
of this commit. Ultimately all three call into lgamma_r, which has heavy
control flow (including switch statements) that would be difficult to
vectorize. Additionally the lgamma_r algorithm is copyrighted to SunPro
so may need a rewrite in the future anyway.
There are no codegen changes (to non-SPIR-V targets) with this commit,
aside from the new half builtins.
d51525ba36