ares_build.h: fix building on 64-bit powerpc

There are two issues.

1. gcc actually does not use __ppc__ and __ppc64__ but __PPC__ and
__PPC64__.  The tests of __ILP32__ and __LP64__ are sufficient for gcc.

2. clang defines __GNU__ and defines both __ppc64__ and __ppc__ when
targeting ppc64.  This makes CARES_SIZEOF_LONG to be 4 on a ppc64 system
when building with clang.

My patch is two change the order of the checks so that we check the
64-bit case first.