MIPS/LD/testsuite: Use $linux_gnu consistently across `mips-elf.exp'

MIPS/LD/testsuite: Use $linux_gnu consistently across `mips-elf.exp'

Replace two remaining spelled-out calls to `[istarget mips*-*-linux*]'
in `mips-elf.exp' with $linux_gnu, which is equivalent.

	ld/
	* testsuite/ld-mips-elf/mips-elf.exp: Replace remaining calls to
	`[istarget mips*-*-linux*]' with $linux_gnu.