x86: Add {rex} pseudo prefix

Add {rex} pseudo prefix to generate a REX byte for integer and legacy
vector instructions if possible.  Note that this differs from the rex
prefix which generates REX prefix unconditionally.

gas/

	* config/tc-i386.c (_i386_insn): Add rex_encoding.
	(md_assemble): When i.rex_encoding is true, generate a REX byte
	if possible.
	(parse_insn): Set i.rex_encoding for {rex}.
	* doc/c-i386.texi: Document {rex}.
	* testsuite/gas/i386/x86-64-pseudos.s: Add {rex} tests.
	* testsuite/gas/i386/x86-64-pseudos.d: Updated.

opcodes/

	* i386-opc.tbl: Add {rex},
	* i386-tbl.h: Regenerated.