MIPS/LD: Fix little-endian `mti' and `img' ELF emulations

Make the little-endian emulation the default for the `mips*el-mti-elf*'
and `mips*el-img-elf*' targets, fixing the issue of LD rejecting, in its
default configuration, object files produced by GAS also in its default
configuration.

	ld/
	* configure.tgt <mips*el-mti-elf*, mips*el-img-elf*>: Add
	targets.