xchg is atomic with side-effects

xchg is atomic with side-effects

clang doesn't know about the side effect, so we have to tell it
that it's okay not to care about the result.

Change-Id: Ib11890bff6779e36cf09c178d224695ea16a8ae8
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/783
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>