The go docs say the exit code is clobbered by `-1` on abnormal exit, but
what they don't say is there's some type mangling along the way so we
see it as 255

Bug: b/144426261
Change-Id: Ife77a950e4739afc94b9f5a950a06f4b853708ae
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/38989
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Tested-by: Chris Forbes <chrisforbes@google.com>