The getOrCreate() method takes care of the mutex locking and unlocking,
and takes the function for creating the routine as a callback argument.

Bug: b/131246679
Change-Id: I0873f6be94c92a11181ce575d1b44ed040177d43
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/42869
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>