[AArch64] Adding ACLE intrinsics for the LS64 extension

This introduces the ARMv8.7-A LS64 extension's intrinsics for 64 bytes
atomic loads and stores: `__arm_ld64b`, `__arm_st64b`, `__arm_st64bv`,
and `__arm_st64bv0`. These are selected into the LS64 instructions
LD64B, ST64B, ST64BV and ST64BV0, respectively.

Based on patches written by Simon Tatham.

Reviewed By: tmatheson

Differential Revision: https://reviews.llvm.org/D93232