Support PREEMPT_DYNAMIC on aarch64

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2065226
Upstream Status: Linux
Tested: Stress tests on aarch64 after runtime changes to preempt
setting.

Support DYNAMIC_PREEMPT for arm64 to match x86_64. RHEL configs
already enables this in common so all that's needed is to enable
it at the arch level.

Signed-off-by: Phil Auld pauld@redhat.com