Racket actually has been ported to all the architectures, just without JIT.

FUTURES actually seems to require JIT (errors with JIT-related object files
when compiling on aarch64), so move FUTURES to the amd64/i386 only section.

aarch64 also required a define. Submitted upstream:

https://github.com/racket/racket/pull/2187

While here, clarify the status on powerpc64 (tested by linimon).

PR: 230019
Submitted by: greg at unrelenting dot technology
Approved by: portmgr (tier-2 blanket)