When creating client threads, we still attempted to invalidate the thread's TLS by writing a null selector to its segment register. In 64-bit mode, this seemed to work most of the time because presumably Intel and AMD are then zero'ing out the the hidden segment's base. Intel doesn't precisely describe this in Vol. 3A 3.4.4.
This patch applies the same scheme introduced and described in #2089 (closed).
Before this patch, the test code_api|client.thread ran into all kinds of assertions, because the client thread interfered with the parent's TLS. These failures are now gone.
Fixes #3526 (closed) Issue: #2089 (closed)