Let the compiler shorten symbols on VMS.

Let the compiler shorten symbols on VMS.

Traditionally Perl does its own name shortening in
ExtUtils::XSSymSet to get around the 31-character limit
on symbol names in the VMS linker.  That method predates
the availability of the /NAMES=SHORTENED option in the
C compiler, which uses the same algorithm as the C++
name mangler to produce shortened symbols.  This change
makes configuration with -Duseshortenedsymbols select
the compiler's shortening over the home-grown shortening.

The advantages of the compiler option over the home-grown
option are that hand-coded long symbols in the core can be
handled (instead of only generated XS_... symbols); long
symbols in external libraries can be handled; and eventually
we can remove XSSymSet and have less to maintain.

84efe3dfc6afbd8ea017ddcc4d5d213cc1a35c72 is required to travel
wherever this change travels for extension building to work
properly.