Opt Compiler: ARM64: Enable explicit memory barriers over acquire/release

Implement remaining explicit memory barrier code paths and temporarily
enable the use of explicit memory barriers for testing.

This CL also enables the use of instruction set features in the ARM64
backend. kUseAcquireRelease has been replaced with PreferAcquireRelease(),
which for now is statically set to false (prefer explicit memory barriers).

Please note that we still prefer acquire-release for the ARM64 Optimizing
Compiler, but we would like to exercise the explicit memory barrier code
path too.

Change-Id: I84e047ecd43b6fbefc5b82cf532e3f5c59076458
Signed-off-by: Serban Constantinescu <serban.constantinescu@arm.com>