Rollup merge of rust-lang#137674 - heiher:enable-f16-loong, r=tgross35

Enable `f16` for LoongArch

Blocked on rust-lang/compiler-builtins#770

r? `@tgross35`

Tracking issue for f16: rust-lang#116909

try-job: dist-loongarch64-linux
try-job: dist-loongarch64-musl

Author: jhpratt

library/std/build.rs

@@ -113,7 +113,6 @@ fn main() {
         // Infinite recursion <https://github.com/llvm/llvm-project/issues/97981>
         ("csky", _) => false,
         ("hexagon", _) => false,
-        ("loongarch64", _) => false,
         ("powerpc" | "powerpc64", _) => false,
         ("sparc" | "sparc64", _) => false,
         ("wasm32" | "wasm64", _) => false,