Fix x build --stage 1 std when using cg_cranelift as the default backend

Before, cg_cranelift would ICE when trying to lower f16 and f128. The
library/ crates had all the infrastructure to omit using them, it just
wasn't hooked up to bootstrap.

src/bootstrap/src/core/build_steps/compile.rs

@@ -523,6 +523,11 @@ pub fn std_cargo(builder: &Builder<'_>, target: TargetSelection, stage: u32, car
 
     let mut features = String::new();
 
+    if stage != 0 && builder.config.default_codegen_backend(target).as_deref() == Some("cranelift")
+    {
+        features += "compiler-builtins-no-f16-f128 ";
+    }
+
     if builder.no_std(target) == Some(true) {
         features += " compiler-builtins-mem";
         if !target.starts_with("bpf") {