MIPSR6: mips-img-elf mips-img-linux-gnu triplets and vendor updates

MIPSR6: mips-img-elf mips-img-linux-gnu triplets and vendor updates

gcc/

	* config.gcc: Support mips*-img-linux* and mips*-img-elf*.
	* config/mips/mti-linux.h: Support mips32r6 as being the default arch.
	* config/mips/t-img-elf: New.
	* config/mips/t-img-linux: New.

gcc/testsuite/

	* gcc.target/mips/pr37362.c: Skip for mips-img-elf.

From-SVN: r218975