x86: Add -mvexwig=[0|1] option to assembler

x86: Add -mvexwig=[0|1] option to assembler

Add -mvexwig=[0|1] option to x86 assembler to control how the assembler
should encode the VEX.W bit in WIG VEX instructions.

	* gas/NEWS: Mention -mvexwig=[0|1] option.
	* config/tc-i386.c (vexwig): New.
	(build_vex_prefix): Set the VEX.W bit for -mvexwig=1 for WIG
	VEX instructions.
	(OPTION_MVEXWIG): New.
	(md_longopts): Add -mvexwig=.
	(md_parse_option): Handle OPTION_MVEXWIG.
	(md_show_usage): Show -mvexwig=[0|1].
	* doc/c-i386.texi: Document -mvexwig=[0|1].
	* testsuite/gas/i386/avx-wig.d: New file.
	* testsuite/gas/i386/avx-wig.s: Likewise.
	* testsuite/gas/i386/avx2-wig.d: Likewise.
	* testsuite/gas/i386/avx2-wig.s: Likewise.
	* testsuite/gas/i386/x86-64-avx-wig.d: Likewise.
	* testsuite/gas/i386/x86-64-avx-wig.s: Likewise.
	* testsuite/gas/i386/x86-64-avx2-wig.d: Likewise.
	* testsuite/gas/i386/x86-64-avx2-wig.s: Likewise.
	* testsuite/gas/i386/i386.exp: Run avx-wig, avx2-wig,
	x86-64-avx-wig and x86-64-avx2-wig.