aarch64: Limit Rt register number for LS64 load/store instructions

Atomic 64-byte load/store instructions limit Rt register number to
values matching below condition (register <Xt> number must be even
and <= 22):

    if Rt<4:3> == '11' || Rt<0> == '1' then UNDEFINED;

This patch adds check if Rt fulfills above requirement.

For more details regarding atomic 64-byte load/store instruction for
Armv8.7 please refer to Arm A64 Instruction set documentation for
Armv8-A architecture profile, see document page 157 for load
instruction, and pages 414-418 for store instructions of [0].

    [0]: https://developer.arm.com/docs/ddi0596/i