arm64: core: force data synchronization between context switch

This patch fixes memory corruption that can happen when running in
multi-thread and multi-core environment with heavy scheduling stress.

In SMP configuration, we must ensure that all thread's context is
stored before writing the switch_handle flag. Otherwise some of the
thread context writes could be delayed after another CPU begins to
schedule this thread which could lead to memory corruptions.

Signed-off-by: Sylvain Chouleur <schouleur@snapchat.com>