Disable f128 math on x86_64

Author: tgross35

library/std/build.rs

@@ -139,11 +139,18 @@ fn main() {
     // LLVM is currenlty adding missing routines, <https://github.com/llvm/llvm-project/issues/93566>
     let has_reliable_f16_math = has_reliable_f16
         && match (target_arch.as_str(), target_os.as_str()) {
+            // Currently nothing special. Hooray!
+            // This will change as platforms gain better better support for standard ops but math
+            // lags behind.
             _ => true,
         };
 
     let has_reliable_f128_math = has_reliable_f128
         && match (target_arch.as_str(), target_os.as_str()) {
+            // LLVM lowers `fp128` math to `long double` symbols even on platforms where
+            // `long double` is not IEEE binary128. See
+            // <https://github.com/llvm/llvm-project/issues/44744>.
+            ("x86_64", _) => false,
             _ => true,
         };