Fix Linux multilib configurations with default architectures

Fix Linux multilib configurations with default architectures

gcc/

	* config/mips/mips.h (MIPS_ISA_LEVEL_SPEC): Only infer an ISA
	level from an ARCH; do not inject the default.
	(MIPS_DEFAULT_ISA_LEVEL_SPEC): New macro split out from
	MIPS_ISA_LEVEL_SPEC.
	(MIPS_ISA_NAN2008_SPEC): Update comment.
	(BASE_DRIVER_SELF_SPECS): Likewise.
	* config/mips/elfoabi.h (DRIVER_SELF_SPECS): Add
	MIPS_DEFAULT_ISA_LEVEL_SPEC.
	* config/mips/mti-elf.h (DRIVER_SELF_SPECS): Likewise.
	* config/mips/mti-linux.h (DRIVER_SELF_SPECS): Likewise.
	* config/mips/sde.h (DRIVER_SELF_SPECS): Likewise.

From-SVN: r219580