aarch64: userspace: Introduce arch_is_user_context

The arch_is_user_context() function is relying on the content of the
tpidrro_el0 register to determine whether we are in user context or not.

This register is set to '1' when in EL1 and set back to '0' when user
threads are running in userspace.

Signed-off-by: Carlo Caione <ccaione@baylibre.com>