Enable f128 tests on all non-buggy platforms 🎉

With the `compiler-builtins` update to 0.1.137 [1], we now provide
symbols necessary to work with `f128` everywhere. This means that we are
no longer restricted to 64-bit linux, and can enable tests by default.

There are still a handful of platforms that need to remain disabled
because of bugs.

Math support is still off by default since those symbols are not yet
available.

[1]: rust-lang#132433

library/std/build.rs

@@ -131,10 +131,10 @@ fn main() {
         ("sparc", _) => false,
         // MinGW ABI bugs <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115054>
         ("x86_64", "windows") if target_env == "gnu" && target_abi != "llvm" => false,
-        // 64-bit Linux is about the only platform to have f128 symbols by default
-        (_, "linux") if target_pointer_width == 64 => true,
-        // Almost all OSs are missing symbol. compiler-builtins will have to add them.
-        _ => false,
+        // There are no known problems on other platforms, so the only requirement is that symbols
+        // are available. `compiler-builtins` provides all symbols required for core `f128`
+        // support, so this should work for everything else.
+        _ => true,
     };
 
     // Configure platforms that have reliable basics but may have unreliable math.