[SubZero] Generate spin-lock for atomic load store

The patch generates LL-SC sequence for atomic load store operations.
64-bit atomic operations are lowered to __sync_*_8 functions.

R=stichnot@chromium.org

Patch from Jaydeep Patil <jaydeep.patil@imgtec.com>.

Review-Url: https://codereview.chromium.org/2682673002 .
4 files changed