[SHELL] Port to AIX

- AIX lacks a WCOREDUMP macro.  It's just used to
  append "(core dumped)" to the crash message, so
  #ifdef around it.

- For some reason, the nl program on AIX defaults
  to not printing line numbers ("-b n"), even though
  the spec says it should default to "-b t".
  Explicitly pass "-b a" for good measure in mkbuiltins.

Signed-off-by: Brian Koropoff <bkoropoff@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>