On Genera running under macOS, do not search /usr/share/common-lisp for ASDF systems:

macOS has implemented a separate read-only system volume and /usr/share is on that volume. Consequently, /usr/share/common-lisp does not exist nor can it be created. Further, Genera accesses its host filesystem through NFS and there is no way to export the root (/) or /usr/share that will not lead Genera into an infinite loop trying to access those mounts. Instead, just search /usr/local/share which is on the writable Data volume.

Also, change the value return by UIOP/OS:ARCHITECTURE to :VLM when running on a VLM.

Intended to replace !159 (closed)

/cc @gpalter