Tweak compat_freebsd32_bit feature name

Mark the current name 'compat_freebsd_32bit' as legacy, and add the
new name 'compat_freebsd32'. This seems to help with some make and
shell uses.

Requested by: jrtc27
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D42641