Reduce MAXPHYS back to 128KB on 32bit architectures.

Some of them have limited KVA, like arm, which prevents startup from
allocating needed number of large pbufs. Other, for instance i386,
are dis-balanced enough after 4/4 that blind bump is probably harmful
because it allows for much more in-flight io than other tunables are
ready for.

Requested by: mmel
Reviewed by: emaste, mmel
Sponsored by: The FreeBSD Foundation