fix my prior hack here to only apply on platforms that need it.
namely, any current netbsd 64 bit platform besides x86.  (i meant
to do this before merging.)

the problem here is the "Node" type is only 4 bytes, an allocation
is assigned to a "Node *node", and then "node + 1" is returned.
this returns a 4-byte aligned pointer, and some callers expect to
be able to store 64-bit (pointer, at least) values there and demand
that the structure be properly aligned.  this change forces Node to
be 8-byte aligned on all-but-x86 64 bit netbsd platforms.