Disable has_reliable_f128_math on musl targets

musl does not implement the symbols required by std for f128 maths.
Disable the associated cfg for all musl targets and adjust the tests
accordingly.

Signed-off-by: Jens Reidel <adrian@travitia.xyz>

compiler/rustc_codegen_llvm/src/llvm_util.rs

@@ -433,6 +433,9 @@ fn update_target_reliable_float_cfg(sess: &Session, cfg: &mut TargetConfig) {
         // This rules out anything that doesn't have `long double` = `binary128`; <= 32 bits
         // (ld is `f64`), anything other than Linux (Windows and MacOS use `f64`), and `x86`
         // (ld is 80-bit extended precision).
+        //
+        // musl does not implement the symbols required for f128 math at all.
+        _ if target_env == "musl" => false,
         ("x86_64", _) => false,
         (_, "linux") if target_pointer_width == 64 => true,
         _ => false,