Fix C compiler flags in the profile self check script (#19326)

This was mistakenly omitted in the original PR.

Author: JukkaL

misc/perf_compare.py

@@ -42,6 +42,8 @@ def build_mypy(target_dir: str, multi_file: bool, *, cflags: str | None = None)
     env["PYTHONHASHSEED"] = "1"
     if multi_file:
         env["MYPYC_MULTI_FILE"] = "1"
+    if cflags is not None:
+        env["CFLAGS"] = cflags
     cmd = [sys.executable, "setup.py", "--use-mypyc", "build_ext", "--inplace"]
     subprocess.run(cmd, env=env, check=True, cwd=target_dir)