Currently only implements the parallelize() function - useful for evenly distributing workloads across threads.

Bug: b/139010488
Change-Id: I7c5705a46afadf137a0cf7fd517b61027ac3a239
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/34819Tested-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>