Preparation for POWER optimizations

Preparation for POWER optimizations

Add the scaffolding for future optimizations for POWER processors. Now
the build is capable of correctly detecting multiple processor
sub-architectures (ppc, ppc64 and ppc64le) and also if features
needed for the optimizations are available during build and runtime.

With these changes, adding a new optimized function for POWER should be
as simple as adding a new file under arch/power/, appending build
instructions to the build files and editing functable.c accordingly.

The UNALIGNED_OK flag is now also added by default for powerpc64le
targets.