x86: Fix atomic_write*() macros to correctly inform GCC that memory

x86: Fix atomic_write*() macros to correctly inform GCC that memory
it knows about is being written to.

The bug is a copy-and-paste error from inline asm that writes to I/O
memory. In that case, as with asm for accessign guest memory,
specifying memory as a read-only parameter is acceptable because the
memory cannot alias with anything that GCC reads directly.

Signed-off-by: Keir Fraser <keir@xen.org>