Add some debugging for emcc --cflags failures. NFC (#20923)

Author: sbc100

emcc.py

@@ -565,6 +565,8 @@ def run(args):
         print(proc.stderr)
         exit_with_error('error getting cflags')
       lines = [x for x in proc.stderr.splitlines() if clang in x and input_file in x]
+      if not lines:
+        exit_with_error(f'unable to parse output of `{cmd}`:\n{proc.stderr}')
       parts = shlex.split(lines[0].replace('\\', '\\\\'))
       parts = [x for x in parts if x not in ['-c', '-o', '-v', '-emit-llvm'] and input_file not in x and temp_target not in x]
       print(shared.shlex_join(parts[1:]))