Clean up the pthread-only atomic stuff a little.

It looks like we can probably just use the generic GCC stuff instead;
the generated code looks pretty similar. We should come back to that.

These routines are only used by the pthread implementation, and
__bionic_atomic_inc isn't used, so we can remove it.

Change-Id: I8b5b8cb30a1b159f0e85c3675aee06ddef39b429