gas: Rename .nop directive to .nops

gas: Rename .nop directive to .nops

Since directives of NO_PSEUDO_DOT targets don't have the leading '.'
and "nop" can be a valid instruction, rename .nop directive to .nops
to avoid conflict.

	* NEWS: Rename .nop to .nops.
	* doc/as.texinfo: Likewise.
	* read.c (potable): Add "nops".  Remove "nop".
	(s_nop): Renamed to ...
	(s_nops): This.
	* read.h (s_nop): Renamed to ...
	(s_nops): This.
	* write.c (cvt_frag_to_fill): Rename .nop to .nops.
	(md_generate_nops): Likewise.
	(relax_segment): Likewise.
	* testsuite/gas/i386/nop-1.d: Updated.
	* testsuite/gas/i386/nop-1.s: Likewise.
	* testsuite/gas/i386/nop-2.d: Likewise.
	* testsuite/gas/i386/nop-2.s: Likewise.
	* testsuite/gas/i386/nop-3.d: Likewise.
	* testsuite/gas/i386/nop-3.s: Likewise.
	* testsuite/gas/i386/nop-4.d: Likewise.
	* testsuite/gas/i386/nop-4.s: Likewise.
	* testsuite/gas/i386/nop-5.d: Likewise.
	* testsuite/gas/i386/nop-5.s: Likewise.
	* testsuite/gas/i386/nop-6.d: Likewise.
	* testsuite/gas/i386/nop-6.s: Likewise.
	* testsuite/gas/i386/nop-bad-1.l: Likewise.
	* testsuite/gas/i386/nop-bad-1.s: Likewise.
	* testsuite/gas/i386/x86-64-nop-1.d: Likewise.
	* testsuite/gas/i386/x86-64-nop-2.d: Likewise.
	* testsuite/gas/i386/x86-64-nop-3.d: Likewise.
	* testsuite/gas/i386/x86-64-nop-4.d: Likewise.
	* testsuite/gas/i386/x86-64-nop-5.d: Likewise.
	* testsuite/gas/i386/x86-64-nop-6.d: Likewise.