Use a few less read-time conditionals

'meta-vmdef' had two variations on creation of an empty bit-vector,
plus an "apology" for doing it that way. This change makes that code
less confusing while making 'compiler/dump' no more confusing.

Also, consecutive reader conditionals are not very portable,
so fixed that in MAYBE-EMIT-MAKE-LOAD-FORMS.
Implement gencgc on ARM.
Enable GENCGC on ARM.