MIPS/LD/testsuite: Make Linux dumps for n64 GOT relocation tests the default

MIPS/LD/testsuite: Make Linux dumps for n64 GOT relocation tests the default

The current default dumps for n64 GOT relocation tests only work for
IRIX targets really, while the Linux dumps are suitable for non-Linux
targets as well.  Make the Linux dumps the default then and handle the
IRIX ones as a special case.

Define an `irix' variable as a shorthand for `[istarget *-*-irix*]' and
use it for these n64 GOT relocation tests as well as export class call
relocation tests.

	ld/
	* testsuite/ld-mips-elf/elf-rel-got-n64.d: Rename to...
	* testsuite/ld-mips-elf/elf-rel-got-n64-irix.d: ... this.
	* testsuite/ld-mips-elf/elf-rel-got-n64-linux.d: Rename to...
	* testsuite/ld-mips-elf/elf-rel-got-n64.d: ... this.
	* testsuite/ld-mips-elf/elf-rel-xgot-n64.d: Rename to...
	* testsuite/ld-mips-elf/elf-rel-xgot-n64-irix.d: ... this.
	* testsuite/ld-mips-elf/elf-rel-xgot-n64-linux.dd: Rename to...
	* testsuite/ld-mips-elf/elf-rel-xgot-n64.d: ... this.
	* testsuite/ld-mips-elf/mips-elf.exp: Update accordingly.
	Define `irix' as a shorthand for `[istarget *-*-irix*]' and use
	it throughout.