pthread: Invalidate stale stack pointers on pthread_exit()

A call to pthread_key_delete() after pthread_exit() have unmapped the stack of a thread
but before the ongoing pthread_join() have finished executing will result in an access
to unmapped memory.
Avoid this by invalidating the stack_base and tls pointers during pthread_exit().

This is based on the investigation and proprosed solution by
Srinavasa Nagaraju <srinavasa.x.nagaraju@sonyericsson.com>

Change-Id: I145fb5d57930e91b00f1609d7b2cd16a55d5b3a9