[AArch64] Fix missing architecture checks for ARMv8.2 system registers.

[Nick Clifton]

Hi Matthew,

> gas/testsuite
> 2016-01-11  Matthew Wahab  <matthew.wahab@arm.com>
>
>      * gas/aarch64/illegal-sysreg-2.l: New.
>      * gas/aarch64/illegal-sysreg-2.d: New.
>
> opcodes/
> 2016-01-11  Matthew Wahab  <matthew.wahab@arm.com>
>
>      * aarch64-opc.c (aarch64_sys_reg_supported_p): Merge conditionals
>      testing for RAS support.  Add checks for erxfr_el1, erxctlr_el1,
>      erxtatus_el1 and erxaddr_el1.

Approved - please apply.

Cheers
   Nick