unittests: add b_lundef=false to address sanitizer test

Clang doesn't really like having no-undefined plus the address sanitizer, but
gcc doesn't mind. This all happens to work with clang + gnu ld, but with clang
+ apple ld this turns into a dumpster fire. Just add b_lundef=false to make
everyone happy.

run_unittests.py

@@ -5032,7 +5032,7 @@ class LinuxlikeTests(BasePlatformTests):
             raise unittest.SkipTest('-fsanitize=address is not supported on OpenBSD')
 
         testdir = os.path.join(self.common_test_dir, '13 pch')
-        self.init(testdir, extra_args=['-Db_sanitize=address'])
+        self.init(testdir, extra_args=['-Db_sanitize=address', '-Db_lundef=false'])
         self.build()
         compdb = self.get_compdb()
         for i in compdb: