[compiler-rt] [builtins] [AArch64] Add missing AArch64 data synchronization...

[compiler-rt] [builtins] [AArch64] Add missing AArch64 data synchronization barrier (dsb) to __clear_cache

https://developer.arm.com/documentation/den0024/a/Caches/Cache-maintenance
covers how to properly clear caches on AArch64, and the builtin
implementation was missing a `dsb ish` after clearing the icache for the
selected range.

Reviewed By: kristof.beyls

Differential Revision: https://reviews.llvm.org/D104094