There's a data race involving threadCount,
so it is now an AtomicInt object.

Also removed unused extern variables.

Bug chromium:778254 chromium:776621

Change-Id: I93174e8f4e5e48f26414ba9c81bb9a2c317dbf73
Reviewed-on: https://swiftshader-review.googlesource.com/13290Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>