[Patch, Fortran] PR32980 - Add GAMMA and LGAMMA intrinsics

[François-Xavier Coudert]

> PPS: I think BUILTIN_GAMMA and BUILTIN_TGAMMA are identically, if
> someone has reason to believe that this is not the case, please tell me.

I'd rather we use tgamma(), which is C99, instead of gamma(), which is not.

FX