Fix getpid caching across a clone.

Fix getpid caching across a clone.

If you make clone, fork, or vfork system calls directly, you're still
on your own, but we now do the right thing for the clone wrapper.
With this implementation, children lose the getpid caching, but we've
no reason to think that that covers any significant use cases.

Bug: 15387103
Change-Id: Icfab6b63c708fea830960742ec92aeba8ce7680d