Auto merge of #99747 - ankane:float_gamma, r=workingjubilee

Add gamma function to f32 and f64

Adds the [gamma function](https://en.wikipedia.org/wiki/Gamma_function) to `f32` and `f64` (`tgamma` and `tgammaf` from C).

Refs:
- rust-lang/rfcs#864
- rust-lang/rust#18271

Author: bors