Tests calling an external function that, itself, calls back into a
Reactor-generated routine.

Bug: b/142132927
Change-Id: I5d89000d4c23804b211aec67d5b7a36778245eba
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/37270Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Tested-by: Antonio Maiorano <amaiorano@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>