xen/arm: make accesses to desc->status flags atomic

xen/arm: make accesses to desc->status flags atomic

This way we don't need to take the desc->lock in order to access
desc->status in many of the gic and vgic functions.

Using *_bit manipulation functions on desc->status is safe on arm64:
status is an unsigned int but is the first field of a struct that
contains pointers, therefore the alignement of the struct is at least 8
bytes.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Julien Grall <julien.grall@linaro.org>