Make libatomic_ops buildable and enable on non-X86.

Change-Id: I9423672b03caa4d500d44155bc47d4a8fa10c3cb
Reviewed-on: https://gerrit.libreoffice.org/5812
Reviewed-by: Andrzej J.R. Hunt <andrzej@ahunt.org>
Tested-by: Andrzej J.R. Hunt <andrzej@ahunt.org>