xen: Introduce cmpxchg64() and guest_cmpxchg64()

xen: Introduce cmpxchg64() and guest_cmpxchg64()

The IOREQ code is using cmpxchg() with 64-bit value. At the moment, this
is x86 code, but there is plan to make it common.

To cater 32-bit arch, introduce two new helpers to deal with 64-bit
cmpxchg().

The Arm 32-bit implementation of cmpxchg64() is based on the __cmpxchg64
in Linux v5.8 (arch/arm/include/asm/cmpxchg.h).

Note that only guest_cmpxchg64() is introduced on x86 so far.

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