rs_fill_nop and md_generate_nops

rs_fill_nop and md_generate_nops

Make rs_fill_nop behave like rs_fill in using a repeat count
(fr_offset) to emit fr_var length repeated nop patterns.  Besides
being more elegant, this reduces memory required for large .nops
directives.

	* as.h (rs_fill_nop): Update comment.
	* config/tc-i386.c (i386_generate_nops): Handle rs_fill_nop as
	for rs_align_code.
	* config/tc-i386.h (MAX_MEM_FOR_RS_SPACE_NOP): Define.
	* listing.c (calc_hex): Handle rs_fill_nop as for rs_fill.
	* read.c (MAX_MEM_FOR_RS_SPACE_NOP): Define.
	(s_nops): Use MAX_MEM_FOR_RS_SPACE_NOP setting up frag.
	* write.c (write_contents): Call md_generate_nops for rs_fill_nop
	before the fr_fix part is written, so that rs_fill_nop can be
	handled as for rs_fill.