platform.uname.processor seems to be what we want in many more cases
than what we were using before — it does the right thing for aarch64,
x86_64, riscv32, riscv64, mips, mips64, powerpc, and powerpc64 (the
latter three of which were broken before).
This fixes cross-compilation of systemd for PowerPC/POWER platforms.
b6304bf11d