Enable reliable_f16_math on x86

This has been disabled due to an LLVM misoptimization with `powi.f16`
[1]. This was fixed upstream and the fix is included in LLVM20, so tests
no longer need to be disabled.

`f16` still remains disabled on MinGW due to the ABI issue.

[1]: llvm/llvm-project#98665

library/std/build.rs

@@ -154,8 +154,6 @@ fn main() {
         && match (target_arch.as_str(), target_os.as_str()) {
             // FIXME: Disabled on Miri as the intrinsics are not implemented yet.
             _ if is_miri => false,
-            // x86 has a crash for `powi`: <https://github.com/llvm/llvm-project/issues/105747>
-            ("x86" | "x86_64", _) => false,
             // Assume that working `f16` means working `f16` math for most platforms, since
             // operations just go through `f32`.
             _ => true,