llvmpipe: Avoid deadlock when unloading opengl32.dll

llvmpipe: Avoid deadlock when unloading opengl32.dll

On Windows, DllMain calls and thread creation/destruction are
serialized, so when llvmpipe is destroyed from DllMain waiting for the
rasterizer threads to finish will deadlock.

So, instead of waiting for rasterizer threads to have finished, simply wait for the
rasterizer threads to notify they are just about to finish.

Verified with this very simple program:

   #include <windows.h>
   int main() {
      HMODULE hModule = LoadLibraryA("opengl32.dll");
      FreeLibrary(hModule);
   }

Fixes https://bugs.freedesktop.org/show_bug.cgi?id=76252

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Cc: 10.2 10.3 <mesa-stable@lists.freedesktop.org>