prune .note.GNU-stack warning from testsuite

binutils/
	* testsuite/lib/binutils-common.exp (prune_warnings_extra): Remove
	.note.GNU-stack warning.
	(run_dump_test): Call prune_warnings for ld and objcopy output.
ld/
	* testsuite/ld-elf/elf.exp: Disable prune_warnings_extra temporarily
	around test for absent .note.GNU-stack
	* testsuite/ld-cris/globsymw2.s,
	* testsuite/ld-cris/warn3.d: Modify "is not implemented" message
	to avoid dejagnu prune_warnings.