[AARCH64][LSE] Preliminary support for ARMv8.1 LSE Atomics.

Implemented support to AArch64 codegen for ARMv8.1 Large System
Extensions atomic instructions. Where supported, these instructions can
provide atomic operations with higher performance.

Currently supported operations include: fetch_add, fetch_or, fetch_xor,
fetch_smin, fetch_min/max (signed and unsigned), swap, and
compare_exchange.

This implementation implies sequential-consistency ordering, more
relaxed ordering is under development.

Subtarget->hasLSE is currently supported for Cavium ThunderX2T99.

Patch by Ananth Jasty.

Differential Revision: https://reviews.llvm.org/D33586

Change-Id: I82f6d3d64255622791ceb0715b7ab9f4dc4d4b2c
llvm-svn: 305893