Allow stubbing of called constant addresses using command line argument.

When specified (via command line) replaces all called constant addresses
with a stubbed call to the first defined function in the bitcode file.
This allows testing of subzero without having to fix that downstream
code (after parsing) may not handle such addresses.

BUG=None
R=jvoung@chromium.org

Review URL: https://codereview.chromium.org/902713002