Restore $(ARCHDIR)vmspipe.com to VMS build after 2d11a7e9678.

That commit moved VMS::Filespec from vms/ext to ext/, but it also
deleted the vmspipe.com dependency from the LIBPREREQ target in
vms/descrip_mms.template.  (vmspipe.com has nothing to do with
VMS::Filespec.)  Which meant vmspipe.com was not availabe for
building extensions or running tests, and -- worse yet -- would
not get installed.  So every pipe creation would involve creating
a temporary version of this file using fallback code in vms/vms.c,
thus making the very pokey pipe implementation even more expensive.
Remove trailing dots in t/porting/readme.t on VMS.

Because on VMS a zero-length extension still has a dot.