MIPS: Use N64 by default for mips*64*-*-linux-gnuabi64

the ABI section of the triple explicitly asks for N64,
and in fact GCC also does so.

It can fix the test failure:
  FAIL: libdep test: did not get expected output from the linker
with Debian's mipsisa64r6el-linux-gnuabi64 toolchain.