f{16,32,64,128}::{erf,erfc}
Also add ```rust // #[unstable(feature = "float_gamma", issue = "99842")] ``` to `gamma`-function-related methods on `f16` & `f128`, as per https://github.com/rust-lang/rust/pull/136324#issuecomment-2626270247
f16
f128
This adds missing functions for math operations on the new float types. Platform support is pretty spotty at this point, since even platforms with generally good support can be missing math functions. `std/build.rs` is updated to reflect this.
