Contains static methods that abstract OS-specific thread / cpu queries and control.

Bug: b/139010488
Change-Id: Idab47624598c3585d7741058ba62974faadc191d
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/34810Tested-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>