MIPS: default r6 if vendor is img

MIPS: default r6 if vendor is img

This behavior is used by downstream toolchain since 2014.
We also set the default ABI for mips*-img-elf to O32.
The previous value is NO_ABI, which is not good default ABI.

We don't support mips64*-img* due to GCC doesn't support it,
and We believe that the multilib should be used for this case.