Enable f16 for LoongArch (#770)

[ the configured-out tests should be re-enabled once we have the symbols
  in nightly - Trevor ]

library/compiler-builtins/configure.rs

@@ -76,10 +76,8 @@ pub fn configure_f16_f128(target: &Target) {
         // Selection failure <https://github.com/llvm/llvm-project/issues/50374>
         "s390x" => false,
         // Infinite recursion <https://github.com/llvm/llvm-project/issues/97981>
-        // FIXME(llvm20): loongarch fixed by <https://github.com/llvm/llvm-project/pull/107791>
         "csky" => false,
         "hexagon" => false,
-        "loongarch64" => false,
         "powerpc" | "powerpc64" => false,
         "sparc" | "sparc64" => false,
         "wasm32" | "wasm64" => false,

library/compiler-builtins/testcrate/build.rs

@@ -75,6 +75,7 @@ fn main() {
         || target.arch == "powerpc"
         || target.arch == "powerpc64"
         || target.arch == "powerpc64le"
+        || target.arch == "loongarch64"
         || (target.arch == "x86" && !target.has_feature("sse"))
         || target.os == "windows"
         // Linking says "error: function signature mismatch: __extendhfsf2" and seems to

library/compiler-builtins/testcrate/tests/conv.rs

@@ -306,7 +306,11 @@ mod extend {
     }
 
     #[cfg(all(f16_enabled, f128_enabled))]
-    #[cfg(not(any(target_arch = "powerpc", target_arch = "powerpc64")))]
+    #[cfg(not(any(
+        target_arch = "powerpc",
+        target_arch = "powerpc64",
+        target_arch = "loongarch64"
+    )))]
     f_to_f! {
         extend,
         f16 => f32, Half => Single, __extendhfsf2, not(feature = "no-sys-f16");
@@ -336,7 +340,11 @@ mod trunc {
     }
 
     #[cfg(all(f16_enabled, f128_enabled))]
-    #[cfg(not(any(target_arch = "powerpc", target_arch = "powerpc64")))]
+    #[cfg(not(any(
+        target_arch = "powerpc",
+        target_arch = "powerpc64",
+        target_arch = "loongarch64"
+    )))]
     f_to_f! {
         trunc,
         f32 => f16, Single => Half, __truncsfhf2, not(feature = "no-sys-f16");