MIPS/GAS/testsuite: Tighten negative-match NaN tests

Fix a test quality regression introduced with commit 351cdf24 [[MIPS]
Implement O32 FPXX, FP64 and FP64A ABI extensions] where MIPS ABI flags
match patterns have been added to negative-match tests covering ELF file
header flags.  Negative-match tests succeed whenever there is a failure
in matching output produced and consequently the likelihood of a false
success increases when patterns to match irrelevant output are added.

Therefore remove the irrelevant paterns so that the tests complete as
soon as the line concerned has been seen.

	gas/testsuite/
	* gas/mips/nan-legacy-1.d: Remove MIPS ABI flags match patterns.
	* gas/mips/nan-legacy-2.d: Likewise.
	* gas/mips/nan-legacy-3.d: Likewise.
	* gas/mips/nan-legacy-4.d: Likewise.
	* gas/mips/nan-legacy-5.d: Likewise.