MIPS: default output r6 obj if the triple is r6

MIPS: default output r6 obj if the triple is r6

If the triple is mipsisa32r6* or mipsisa64r6*, ld/as should output
r6 objects by default.
The triples with vendor `img` should do same.

The examples include:
	as xx.s -o xx.o
	ld -r -b binary xx.dat -o xx.o