Add Power8 VSX SIMD support

This adds the low-level SIMD implementation
for IBM VMX, which is present on both
Power7 and Power8. It passes unit tests with
both gcc-4.9 and IBM xlc on Power7 (which is
always big endian), and with gcc 4.8 and 4.9
on Power8 running linux in little-endian mode.
It is not yet enabled automatically since we
still lack nbnxn kernels for this architecture.

Extended the Doxygen suppressions for gmx_cpuid() to the whole file,
since wildcards in suppressions.txt were not working, we we don't
understand what doxygen bug is responsible, and there's not actually
any Doxygen documentation in that file.

Change-Id: I06570992cd3a98c0debf5ff25100d68d55d0f8fb