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: NChrysostomos Nanakos <cnanakos@grnet.gr>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>