x86: Drop using extension_opcode to encode vvvv register

gas/ChangeLog:

        * config/tc-i386.c (build_modrm_byte): Dropped the use of
        extension_opcode to encode the vvvv register.
        * testsuite/gas/i386/x86-64-sse2avx.d: Added new testcases.
        * testsuite/gas/i386/x86-64-sse2avx.s: Diito.

opcodes/ChangeLog:

        * i386-opc.tbl: Added DstVVVV to some extension_opcode instructions.
	* i386-tbl.h: Regenerated.