ld: support most modifiers for printf() like vfinfo() format specifiers

Not doing so is pretty error prone: One needs to distinguish e.g.
->einfo() and alike invocations out of libbfd from _bfd_error_handler()
ones. Omit support for * though for now, as that would be more intrusive
to implement.

Use this then to disambiguate x86'es ISA level diagnostic of unknown bits,
where decimal vs hex isn't immediately clear.