xen/arm: atomic: Rewrite write_atomic()

xen/arm: atomic: Rewrite write_atomic()

The current implementation of write_atomic has two issues:
    1) It cannot be used to write pointer value because the switch
    contains cast to other size than the size of the pointer.
    2) It will happily allow to write to a pointer to const.

Additionally, the Arm implementation is returning a value when the x86
implementation does not anymore. This was introduced in commit
2934148a0773 "x86: simplify a few macros / inline functions". There are
no users of the return value, so it is fine to drop it.

The switch is now moved in a static inline helper allowing the compiler
to prevent use of const pointer and also allow to write pointer value.

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