A synchronization primitive borrowed from golang:
https://golang.org/pkg/sync/#WaitGroup

Bug: b/133127573
Change-Id: Id3ec093c0de478cfe16fad4d064cf2c26288acf7
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/31680Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>