[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