support rust installed without rustup

tools/rust-lld-wrapper

@@ -45,7 +45,10 @@ def find_host_triplet():
     return host.group(1)
 
 def find_rust_lld():
-    which = subprocess.run(["rustup", "which", "rustc"], stdout=subprocess.PIPE, stderr=subprocess.PIPE)
+    try:
+        which = subprocess.run(["rustup", "which", "rustc"], stdout=subprocess.PIPE, stderr=subprocess.PIPE)
+    except FileNotFoundError:
+        return "lld"
     which.check_returncode()
 
     rustc_path = which.stdout.decode("utf8").strip()