MIPS/LD/testsuite: Accept any UNIX OS/ABI with GNU attribute tests

Remove failures with FreeBSD targets, e.g:

regexp_diff match failure
regexp "^  OS/ABI:                            UNIX - System V$"
line   "  OS/ABI:                            UNIX - FreeBSD"
FAIL: ld-mips-elf/attr-gnu-4-50

introduced with commit 351cdf24 ("[MIPS] Implement O32 FPXX, FP64
and FP64A ABI extensions").

	ld/
	* testsuite/ld-mips-elf/attr-gnu-4-10.d: Match any UNIX OS/ABI.
	* testsuite/ld-mips-elf/attr-gnu-4-50.d: Likewise.
	* testsuite/ld-mips-elf/attr-gnu-4-60.d: Likewise.
	* testsuite/ld-mips-elf/attr-gnu-4-70.d: Likewise.