MIPS: Use 64-bit a ABI by default for `mipsisa64*-*-linux*' targets

MIPS: Use 64-bit a ABI by default for `mipsisa64*-*-linux*' targets

Following the arrangement in GCC select a 64-bit ABI by default, either
n32 or n64, rather than o32 for `mipsisa64*-*-linux*' targets, just as
with the corresponding `mips64*-*-linux*' targets.