Instead of exposing the endianness in the CPU family, canonicalise the CPU
family to just "ppc64" to match MIPS (which is also bi-endian).
Part of the work for #3842.
* environment: validate cpu_family in cross file
* run_unittests: add unittest to ensure CPU family list in docs and environment matches
* run_unittests: skip compiler options test if not in a git repository
* environment: validate the detected cpu_family
* docs: add 32-bit PowerPC and 32/64-bit MIPS to CPU Families table
Names gathered by booting Linux in Qemu and running:
$ python3
import platform; platform.machine()
Partial fix for #3751