rustbuild: Allow create_sysroot in stage0

Despite what the comment says, we actually need to do this. We're not cleaning
out the stage0 compiler's sysroot, but rather just our own sysroot that we
assembled previously.

src/bootstrap/compile.rs

@@ -258,11 +258,6 @@ fn compiler_file(compiler: &Path, file: &str) -> PathBuf {
 }
 
 pub fn create_sysroot(build: &Build, compiler: &Compiler) {
-    // nothing to do in stage0
-    if compiler.stage == 0 {
-        return
-    }
-
     let sysroot = build.sysroot(compiler);
     let _ = fs::remove_dir_all(&sysroot);
     t!(fs::create_dir_all(&sysroot));