Add -momit_lock_prefix=[no|yes] option

Add -momit_lock_prefix=[no|yes] option

This option serves as a workaround for processors, which fail on lock
prefix.

gas/
	* config/tc-i386.c (omit_lock_prefix): New.
	(output_insn): Omit lock prefix if omit_lock_prefix is true.
	(OPTION_omit_lock_prefix): New.
	(md_longopts): Add momit-lock-prefix.
	(md_parse_option): Handle momit-lock-prefix.
	(md_show_usage): Add momit-lock-prefix=[no|yes].
	* doc/c-i386.texi (momit-lock-prefix): Document.

gas/testsuite/

	* gas/i386/i386.exp: Run new tests.
	* gas/i386/omit-lock-no.d: New.
	* gas/i386/omit-lock-yes.d: Ditto.
	* gas/i386/omit-lock.s: Ditto.