Make the JS collector work with multiple threads

Make the JS collector work with multiple threads

Reviewed by Maciej and Darin.

Under heavy contention it was possible the GC to suspend other
threads inside the pthread spinlock, which could lead to the GC
thread blocking on the pthread spinlock itself.

We now determine and store each thread's stack base when it is
registered, thus removing the need for any calls to pthread_get_stackaddr_np
that needed the pthread spinlock.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@28223 268f45cc-cd09-0410-ab3c-d52691b4dbfc