Infer architecture from ABI for mips-mti* and mips-img* triplets.

Infer architecture from ABI for mips-mti* and mips-img* triplets.

gcc/
	* config.gcc (mips*-mti-elf*, mips*-mti-linux*): Set mips32r2
	and mips64r2 as default 32-bit and 64-bit architectures.
	(mips*-img-elf*, mips*-img-linux*): Set mips32r6 and mips64r6
	as default 32-bit and 64-bit architectures.

From-SVN: r240145