[AArch64] Refactor LSE support as feature separate from V8.1a support.

Summary:
This is preparation for ThunderX processors that have Large
System Extension (LSE) atomic instructions, but not the 
other instructions introduced by V8.1a.
This will mimic changes to GCC as described here:
https://gcc.gnu.org/ml/gcc-patches/2015-06/msg00388.html

LSE instructions are: LD/ST<op>, CAS*, SWP

Reviewers: t.p.northover, echristo, jmolloy, rengolin

Subscribers: aemerson, mehdi_amini

Differential Revision: https://reviews.llvm.org/D26621

llvm-svn: 288279