[Aarch64] Support ARMv8.2 AT instructions

[Aarch64] Support ARMv8.2 AT instructions

ARMv8.2 adds new instructions AT S1E1RP and AT S1E1WP to Aarch64. This
patch adds support for the instructions, making them available when
-march=armv8.2-a is selected.

gas/testsuite/
2015-12-10  Matthew Wahab  <matthew.wahab@arm.com>

	* gas/aarch64/sysreg-2.d: Update for new tests for AT S1E1RP and
	AT S1E1WP.
	* gas/aarch64/sysreg-2.s: Add tests for AT S1E1RP and AT S1E1WP.

opcodes/
2015-12-10  Matthew Wahab  <matthew.wahab@arm.com>

	* aarch64-opc.c (aarch64_sys_regs_at): Add "s1e1rp" and "s1e1wp".
	(aarch64_sys_ins_reg_supported_p): Add ARMv8.2 system register
	feature test for "s1e1rp" and "s1e1wp".

Change-Id: I09e1044b629ab0a34b03c423e8d4e71ff92daad4