Enable f16 tests on non-GNU Windows

There is a MinGW ABI bug that prevents `f16` and `f128` from being
usable on `windows-gnu` targets. This does not affect MSVC; however, we
have `f16` and `f128` tests disabled on all Windows targets.

Update the gating to only affect `windows-gnu`, which means `f16` tests
will be enabled. There is no effect for `f128` since the default
fallback is `false`.

Author: tgross35

library/std/build.rs

@@ -104,7 +104,7 @@ fn main() {
         // Unsupported <https://github.com/llvm/llvm-project/issues/94434>
         ("arm64ec", _) => false,
         // MinGW ABI bugs <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115054>
-        ("x86_64", "windows") => false,
+        ("x86_64", "windows") if target_env == "gnu" => false,
         // Apple has a special ABI for `f16` that we do not yet support
         // FIXME(builtins): fixed by <https://github.com/rust-lang/compiler-builtins/pull/675>
         ("x86" | "x86_64", _) if target_vendor == "apple" => false,
@@ -135,10 +135,10 @@ fn main() {
         // ABI unsupported  <https://github.com/llvm/llvm-project/issues/41838>
         ("sparc", _) => false,
         // MinGW ABI bugs <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115054>
-        ("x86_64", "windows") => false,
+        ("x86_64", "windows") if target_env == "gnu" => false,
         // 64-bit Linux is about the only platform to have f128 symbols by default
         (_, "linux") if target_pointer_width == 64 => true,
-        // Same as for f16, except MacOS is also missing f128 symbols.
+        // Almost all OSs are missing symbol. compiler-builtins will have to add them.
         _ => false,
     };