bionic: Add shadow call stack guard region field to pthread_internal_t.

Landing this change separately to the change that implements SCS
because it needs to land at the same time as an internal change. This
will simplify the situation in case SCS needs to be reverted again.

Change-Id: Ibe18750829665b6dcf6e36628a5e5bbdd1a0dd4f
Merged-In: Ibe18750829665b6dcf6e36628a5e5bbdd1a0dd4f