objtool: Exit successfully when requesting help

When the user requests help it's not an error so do not exit with
a non-zero exit code. This is not especially useful for a user but
any script that might wish to check that objtool --help is at least
available can't rely on the exit code to crudely check that, for
example, building an objtool executable succeeds.

Signed-off-by: Matt Helsley <mhelsley@vmware.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>