Disable o32 ABI for MIPS64 architectures

Building o32 ELF files for MIPS64 is an exotic configuration that nobody
should be using. If o32 is required, then is better if it's built for
MIPS 32-bit cores so only 32-bit instructions will be used leading to a
more efficient o32 usage.

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>