[AArch64] Add support for Transactional Memory Extension (TME)

TME is a future architecture technology, documented in

https://developer.arm.com/architectures/cpu-architecture/a-profile/exploration-tools
https://developer.arm.com/docs/ddi0601/a

More about the future architectures:

https://community.arm.com/developer/ip-products/processors/b/processors-ip-blog/posts/new-technologies-for-the-arm-a-profile-architecture

This patch adds support for the TME instructions TSTART, TTEST, TCOMMIT, and
TCANCEL and the target feature/arch extension "tme".

It also implements TME builtin functions, defined in ACLE Q2 2019
(https://developer.arm.com/docs/101028/latest)

Patch by Javed Absar and Momchil Velikov

Differential Revision: https://reviews.llvm.org/D64416

llvm-svn: 366322