Auto merge of #135074 - wzssyqa:mips-mti, r=oli-obk

Target: Add mips mti baremetal support

Do the same thing as gcc, which use the vendor `mti` to mark the toolchain as MIPS32r2 default.

We support both big endian and little endian flavor:
  mips-mti-none-elf
  mipsel-mti-none-elf

Author: bors