[mypyc] Make C unit tests faster by compiling with -O0 (#17884)

Most time is spent in compilation, so runtime performance doesn't really
matter.

mypyc/test/test_external.py

@@ -20,7 +20,9 @@ def test_c_unit_test(self) -> None:
         cppflags: list[str] = []
         env = os.environ.copy()
         if sys.platform == "darwin":
-            cppflags += ["-mmacosx-version-min=10.10", "-stdlib=libc++"]
+            cppflags += ["-O0", "-mmacosx-version-min=10.10", "-stdlib=libc++"]
+        elif sys.platform == "linux":
+            cppflags += ["-O0"]
         env["CPPFLAGS"] = " ".join(cppflags)
         # Build Python wrapper for C unit tests.