block/archipelago: Use QEMU atomic builtins Replace __sync builtins with ones provided by QEMU for atomic operations. Special thanks goes to Paolo Bonzini for his refactoring suggestion in order to use the already existing atomic builtins interface. Signed-off-by: Chrysostomos Nanakos <cnanakos@grnet.gr> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>