Force assignment to read the old pointer value twice, and check
that it didn't change in the interim. Previous experience with
Skia suggests that this has a high probability of correctly detecting
a data race when it occurs, instead of potentially letting the
count associated with the old pointer value get decremented twice,
and corrupting the heap.
This does increase the size of sp assignments, which seem to
commonly get inlined. For the general case, we add a third
comparison and function call.
Some code reformatting to make this consistent with modern conventions
and pass automated checks.
Test: Booted aosp build. Ran libutils tests. Looked at generated code.
Bug: 31227650
Change-Id: Id93a05c6bf10f01ee15ff1bb409611f2058f988f
