[AArch64] LSE Atomics reorg - part 1

Add memory synchronization semantics to LSE Atomics.

The memory semantics feature will be added in a subsequent patch.

In this patch, several corrections were added to the existing LSE Atomics
implementation, based on the ARM Errata D11904 from 05/12/2017.

Patch by: steleman

Differential Revision: https://reviews.llvm.org/D35319

llvm-svn: 310167