x86: Support x86 Zhaoxin PadLockRNG2 instruction

This patch adds support for Zhaoxin PadLock RNG2 instruction, the
CPUID EDX bit[23] indicates its enablement, it includes REP XRNG2
instruction.

gas/ChangeLog:

	* NEWS: Support Zhaoxin PadLock RNG2 instruction.
	* config/tc-i386.c (add_branch_prefix_frag_p): Don't add prefix to
	PadLock RNG2 instruction.
	(output_insn): Handle PadLock RNG2 instruction.
	* doc/c-i386.texi: Document PadLock RNG2.
	* testsuite/gas/i386/i386.exp: Add PadLock RNG2 test.
	* testsuite/gas/i386/padlock_rng2.d: Ditto.
	* testsuite/gas/i386/padlock_rng2.s: Ditto.

opcodes/ChangeLog:

	* i386-dis.c: Add PadLockRNG2.
	* i386-gen.c: Ditto
	* i386-opc.h (CpuPadLockRNG2): New.
	* i386-opc.tbl: Add Zhaoxin PadLock RNG2 instruction.
	* i386-tbl.h: Regenerated.
	* i386-mnem.h: Ditto.
	* i386-init.h: Ditto.