xen/arm: atomic: Allow read_atomic() to be used in more cases

xen/arm: atomic: Allow read_atomic() to be used in more cases

The current implementation of read_atomic() on Arm will not allow to:
    1) Read a value from a pointer to const because the temporary
    variable will be const and therefore it is not possible to assign
    any value. This can be solved by using a union between the type and
    a char[0].
    2) Read a pointer value (e.g void *) because the switch contains
    cast from other type than the size of a pointer. This can be solved by
    by introducing a static inline for the switch and use void * for the
    pointer.

Reported-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>