Okay, so to be clear, I am assuming XLC supports all of the following fields, and uses unsigned bit fields by default:

struct UNSIGNED_BITS {
    unsigned int AU: 1;
    int A: 1; /* Equivalent to unsigned int */
    signed int AS: 1;
    unsigned short MU: 1;
    short M: 1; /* Equivalent to unsigned short; triggers warning */
};

and that it cannot compile the following:

struct SIGNED_BITS {
    signed short MS: 1; /* Not supported */
};

Attached is what I think a patch to resolve this would look like. However it needs a line in Modules/_ctypes/_ctypes_test.c completed to detect the compiler:

#ifndef /* Something to identify XLC */

Can you figure out a way to test for XLC (but not GCC, which the AIX buildbot uses), and then try my patch out? Hopefully you see no more compiler warnings, test_ints() should now pass, and test_shorts() should be skipped.