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