asm/atomic.h: common prototyping (add xen/atomic.h)

asm/atomic.h: common prototyping (add xen/atomic.h)

Create a common-side <xen/atomic.h> to establish, among others, prototypes of
atomic functions called from common-code. Done to avoid introducing
inconsistencies between arch-side <asm/atomic.h> headers when we make subtle
changes to one of them. Some arm-side macros had to be turned into inline
functions in the process.

Removed outdated comment ("NB. I've [...]").

Signed-off-by: Corneliu ZUZU <czuzu@bitdefender.com>
Suggested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Acked-by: Julien Grall <julien.grall@arm.com>