[AArch64] Add support for ARMv8.1 Virtulization Host Extensions.

The ARMv8.1 architecture includes the Virtualization Host Extensions
which add a number of system registers. This patch adds support for
these system registers, making them available when -march=armv8.1-a is
selected.

include/opcode/
2015-11-20  Matthew Wahab  <matthew.wahab@arm.com>

	* aarch64.h (AARCH64_FEATURE_V8_1): New.
	(AARCH64_ARCH_v8_1): Add AARCH64_FEATURE_V8_1.

opcodes/
2015-11-20  Matthew Wahab  <matthew.wahab@arm.com>

	* aarch64-opc.c (aarch64_sys_regs): Add spsr_el12, elr_el12,
	sctlr_el12, cpacr_el12, ttbr1_el2, ttbr0_el12, ttbr1_el12,
	tcr_el12, afsr0_el12, afsr1_el12, esr_el12, far_el12, mair_el12,
	amair_el12, vbar_el12, contextidr_el2, contextidr_el12,
	cntkctl_el12, cntp_tval_el02, cntp_ctl_el02, cntp_cval_el02,
	cntv_tval_el02, cntv_ctl_el02, cntv_cval_el02, cnthv_tval_el2,
	cnthv_ctl_el2, cnthv_cval_el2.
	(aarch64_sys_reg_supported_p): Update for the new system
	registers.

gas/testsuite/
2015-11-20  Matthew Wahab  <matthew.wahab@arm.com>

	* gas/aarch64/virthostext-directive.d: New.
	* gas/aarch64/virthostext.d: New.
	* gas/aarch64/virthostext.s: New.

Change-Id: Iecb370591b1b6e9e00d81c8ccd9ae3b0f71794a2