Enable f16 on platforms that were missing conversion symbols

The only requirement for `f16` support, aside from LLVM not crashing and
no ABI issues, is that symbols to convert to and from `f32` are
available. Since the update to compiler-builtins in [1], we now provide
these on all platforms.

This also enables `f16` math since there are no further requirements.

[1]: rust-lang#125016

library/std/build.rs

@@ -106,18 +106,10 @@ fn main() {
  // x86 has ABI bugs that show up with optimizations. This should be partially fixed with
  // the compiler-builtins update. <https:/rust-lang/rust/issues/123885>
  ("x86" | "x86_64", _) => false,
- // Missing `__gnu_h2f_ieee` and `__gnu_f2h_ieee`
- ("powerpc" | "powerpc64", _) => false,
- // Missing `__gnu_h2f_ieee` and `__gnu_f2h_ieee`
- ("mips" | "mips32r6" | "mips64" | "mips64r6", _) => false,
- // Missing `__extendhfsf` and `__truncsfhf`
- ("riscv32" | "riscv64", _) => false,
- // Most OSs are missing `__extendhfsf` and `__truncsfhf`
- (_, "linux" | "macos") => true,
- // Almost all OSs besides Linux and MacOS are missing symbols until compiler-builtins can
- // be updated. <https:/rust-lang/rust/pull/125016> will get some of these, the
- // next CB update should get the rest.
- _ => false,
+ // `f16` support only requires that symbols converting to and from `f32` are available. We
+ // provide these in `compiler-builtins`, so `f16` should be available on all platforms that
+ // do not have other ABI issues or LLVM crashes.
+ _ => true,
  };
 
  let has_reliable_f128 = match (target_arch.as_str(), target_os.as_str()) {