Replace sw::MutexLock with std::mutex.
Replace use of sw::Thread with std::thread.

helgrind seems to take objection to the init Event.

AtomicInt remains as a wrapper around std::atomic<int> as it provides an 'operator=' and
'std::atomic& operator=( const std::atomic& )' is deleted.

This change is to reduce the amount of code I have to verify while looking for synchronization issues.

Bug: b/133127573
Change-Id: I8cbe38c918298ffafaf59fe25b0ff258670f9a99
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/31812Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>