Use a standard pointer-sized integer type
Fix the __get_cpuid cmake check
Check that atomic_load works with a const _Atomic

The original C11 spec does not allow atomic_load to work on const _Atomic
variables, which we sometimes do in alGet* methods, despite the fact that it
does not modify the variable. An update to the C spec corrects this oversight,
and GCC 4.9 has allowed it anyway, while Clang 3.4 does not.