Eliminate TSan false positive related to stop_info.stack_ptr access

Eliminate TSan false positive related to stop_info.stack_ptr access

* pthread_stop_world.c [!GC_OPENBSD_UTHREADS && !NACL]
(GC_store_stack_ptr): New inline function definition (move the code
from GC_suspend_handler_inner but stop_info.stack_ptr is now stored
atomically); add comment.
* pthread_stop_world.c [!GC_OPENBSD_UTHREADS && !NACL]
(GC_suspend_handler_inner): Call GC_store_stack_ptr().
* pthread_stop_world.c [!GC_OPENBSD_UTHREADS && !NACL]
(GC_push_all_stacks): Use AO_load to fetch p->stop_info.stack_ptr.