objtool: Resurrect --backup option

objtool: Resurrect --backup option

The --backup option was removed with the following commit:

  aa8b3e64fd39 ("objtool: Create backup on error and print args")

... which tied the backup functionality to --verbose, and only for
warnings/errors.

It's a bit inelegant and out of scope to tie that to --verbose.

Bring back the old --backup option, but with the new behavior: only on
warnings/errors, and print the args to make it easier to recreate.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Acked-by: Petr Mladek <pmladek@suse.com>
Tested-by: Joe Lawrence <joe.lawrence@redhat.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>