thread_local support for AIX

Summary:
The AIX linker will produce errors on unresolved weak symbols.  Change the
generated code to not check for the initialization function but just call
it and ensure that it always exists.  Also, the AIX atexit routine has a
different name (and signature) so call it correctly.  Update the lit tests
to test on AIX appropriately.

Author: Jamie Schmeiser <schmeise@ca.ibm.com>
Reviewed By: hubert.reinterpretcast (Hubert Tong)
Differential Revision: https://reviews.llvm.org/D104420