These simple changes are required to allow builds on ppc64[le] systems
consistent with X86. The Makefile now recognizes both ppc64 and ppc64le, and
in the absence of PORTABLE=1, the code will be built analogously to the X86
-march=native.

Note that although GCC supports -mcpu=native -mtune=native on POWER, it
doesn't work correctly on all systems. This is why we need to get the actual
machine model from the AUX vector.