Do not merge: change fallback for testing

library/std/build.rs

@@ -116,10 +116,9 @@ fn main() {
         ("riscv32" | "riscv64", _) => false,
         // Most OSs are missing `__extendhfsf` and `__truncsfhf`
         (_, "linux" | "macos") => true,
-        // Almost all OSs besides Linux and MacOS are missing symbols until compiler-builtins can
-        // be updated. <https://github.com/rust-lang/rust/pull/125016> will get some of these, the
-        // next CB update should get the rest.
-        _ => false,
+        // todo: only for testing, this line will wind up changed as part of
+        // <https://github.com/rust-lang/rust/pull/129385>.
+        _ => true,
     };
 
     let has_reliable_f128 = match (target_arch.as_str(), target_os.as_str()) {