Stub all functions unimplemented by the subzero backend.

Change-Id: I9dd3a1f7dd633934f4ee76d6807a89a4b117232a
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/28849
Presubmit-Ready: Ben Clayton <bclayton@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>