Fix invalid TestBitfields case (thanks Filipe!), and xfail one case due to GCC compiler bug.

Turns out unnamed bit fields should not be accessed ever in C (C99 section 4.7.8) or C++
(C++11 section 9.6).

Unrelated to the above, this commit marks TestBitfields as expected-to-fail with GCC as 4.7
has problems with padding... the size of the struct with a uint32_t member is reported as 12
when GCC is used. Clang emits correct code.

Parts of this patch prepared by Filipe Cabecinhas!