[PATCH][GAS] aarch64: Add atomic 64-byte load/store instructions for Armv8.7

Armv8.7 architecture introduces the "accelerator extension", aka
load/store of 64 bytes. New atomic load/store instructions are: LD64B,
ST64B, ST64BV and ST64BV0.

This patch adds:
+ New feature +ls64 to -march command line.
+ New atomic load/store instructions associated with above feature.

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