Use full archname in architecture-specific directories on VMS.

Use full archname in architecture-specific directories on VMS.

We were only using the base archname (e.g., "VMS_AXP"), which is
ok for a default bulid, but if any additions were made to it,
such as "-thread", there would be a discrepancy between the
actual directory on disk and what perl.c:S_incpush would look for
when loading up @INC.  The net effect was that the architecture-
specific directory would not get loaded into @INC.