cmd/link: use standard library flag package where possible
The obj library's flag functions are (mostly) light wrappers
around the standard library flag package. Use the flag package
directly where possible.
Most uses of the 'count'-type flags (except for -v) only check
against 0, so they can safely be replaced by bools. Only -v
and the flagfns haven't been replaced.
Debug has been turned into a slice of bools rather than ints.
There was a copy of the -v verbosity in ctxt.Debugvlog, so don't use
Debug['v'] and just use ctxt.Debugvlog.