Initialization of thread_local variables in shared libraries might be
implementation dependent.

Just keep the assert for Win32.
https://docs.microsoft.com/en-us/cpp/cpp/thread?view=vs-2019 states that
"Thread-local variables that are initialized statically with constant
values are generally initialized properly on all threads."

Bug: b/157525646
Change-Id: Ifb6a77e085c0f21f13af58c89050868992928300
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/45809
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>