gh-90548: Skip ctypes test_null_dlsym when linked to musl (GH-138592)

The test relies on glibc-specific behavior.
(cherry picked from commit 7d58461)

Co-authored-by: Zachary Ware <[email protected]>

Author: zware

Lib/test/test_ctypes/test_dlerror.py

@@ -32,6 +32,7 @@
 
 @unittest.skipUnless(sys.platform.startswith('linux'),
                      'test requires GNU IFUNC support')
+@unittest.skipIf(test.support.linked_to_musl(), "Requires glibc")
 class TestNullDlsym(unittest.TestCase):
     """GH-126554: Ensure that we catch NULL dlsym return values