Remove unused DMB code paths in the ARM64 Optimizing Compiler

Currently all ARM64 CPUs will be using the acquire-release code paths.
This patch removes the instruction set feature PreferAcquireRelease()
as well as all the unused DMB code paths.

Change-Id: I61c320d6d685f96c9e260f25eac3593907793830
Signed-off-by: Serban Constantinescu <serban.constantinescu@linaro.org>
4 files changed