[obish] Convert attach.exp to gdb_test_multiple

[Michael Elizabeth Chastain]

> I'll leave committing this to tomorrow (i.e., after the branch date). 
> Still passes on amd64 GNU/Linux and PPC NetBSD.

Hey, we don't have any ppc netbsd results in gdb-testers@.

Can I get you to send an abridged gdb.sum file to gdb-testers?

  grep -v '^PASS:' gdb.sum
  add a couple lines at the top:
    netbsd/ppc
    kernel version number
    compiler name and version number
  mail to gdb-testers@

Michael C