Enable f16 for MIPS

It seems as if `f16` works on MIPS now according to my
testing on Rust master with LLVM 20, and I was asked to
create PRs with my changes.

I only tested on the flavour of `mipsel-unknown-linux-gnu`
hardware that happens to be available to me, so I can't say
anything about other MIPS hardware, but from a casual
skimming of the LLVM code ([1], [2]) it seems like `f16`
should work on all MIPS hardware. So enable it for all MIPS
hardware.

[1]: https://github.com/rust-lang/llvm-project/blob/rustc/20.1-2025-02-13/llvm/lib/Target/Mips/MipsISelLowering.h#L370
[2]: https://github.com/rust-lang/llvm-project/blob/rustc/20.1-2025-02-13/llvm/lib/CodeGen/TargetLoweringBase.cpp#L1367-L1388

library/std/build.rs

@@ -107,7 +107,6 @@ fn main() {
         ("csky", _) => false,
         ("hexagon", _) => false,
         ("loongarch64", _) => false,
-        ("mips" | "mips64" | "mips32r6" | "mips64r6", _) => false,
         ("powerpc" | "powerpc64", _) => false,
         ("sparc" | "sparc64", _) => false,
         ("wasm32" | "wasm64", _) => false,