This was added so that the new thread definitely has its threadid set, via
SDL_GetCurrentThreadID(), before SDL_CreatThread returns, but this broke
Emscripten, which can't wait on a newly-created thread, since the thread won't
start until a later mainloop iteration.
Now we have the creating thread set this id in SDL_SYS_CreateThread, where
platform-specific logic can figure out how to calculate the new thread's ID
from the parent thread, without using SDL_GetCurrentThreadID().
Fixes#15509.
(cherry picked from commit 922b872b4f)
6d35168a63