Fix DATASTART (and other minor improvements) for NaCl target

Fix DATASTART (and other minor improvements) for NaCl target

* include/private/gcconfig.h (DATASTART): Define to a fixed address
for NaCl.
* pthread_stop_world.c (GC_suspend_all): Issue GC warning if it
appears stalled waiting for some threads to park (NaCl only).
* pthread_stop_world.c (nacl_register_gc_hooks): Declare external
function (available from NaCl runtime).
* pthread_stop_world.c (GC_nacl_initialize_gc_thread): Invoke
nacl_register_gc_hooks registring nacl_pre_syscall_hook and
nacl_post_syscall_hook (NaCl only).