net/viamillipede: mark as NOT_FOR_ARCHS=armv6 armv7

Seems like dtrace doesn't work on arm:

dtrace: failed to link script dtrace_viamillipede.d: an error was encountered while processing plumbing.o
dt_modtext:/usr/local/poudriere/jails/13Rarmv7/usr/src/cddl/contrib/opensolaris/lib/libdtrace/common/dt_link.c(843): arm not implemented

Approved by: portmgr (build fix blanket)