Allow MIPS lib32 to build with clang.

Don't try to set -march as clang doesn't permit use of the O32 ABI
with newer CPU microarchitectures like 'mips3'. In addition, clang
doesn't permit the O32 ABI with the default N64 target, so use an
explicit O32 -target for clang.

Sponsored by: DARPA / AFRL