Mark www/node as buildable by powerpc64

Summary:
As of node 9.3.0, a bug was fixed preventing builds with gcc. As this has
not been tested on mips, and fails on powerpcspe, only add powerpc64 to the
ONLY_FOR_ARCHS list.

Test Plan: Built for powerpc64

Reviewed By: bhughes
Differential Revision: https://reviews.freebsd.org/D13880