[build] -mabi=64 for mips/ppc64 with --m=64

Do the right default for --m=64 on ppc/mips with gcc multilib.
These default to 32-bit, and need -mabi=64.
See GH #1110.

tested only on mips64 so far. ppc64 later, arm64 (aarch64) probably
also. sparc64 is different.