Add check for pthread_self() when looking up a thread

Check if thread_id is in fact pthread_self before
locking on g_thread_list_lock in __pthread_internal_find.

The main reason for doing this is not performance but to allow
the linker use raise() which was not working because pthread_kill()
couldn't find pthread_self() thread because the global thread
list is initialized in libc.so and the linker's version of this
list is empty.

Bug: http://b/25867917
Change-Id: I18fe620e8cd465b30f0e1ff45fff32958f3c5c00