Make compiler-shortened symbols the default on VMS.

Make compiler-shortened symbols the default on VMS.

It used to be that once or twice per decade a symbol longer than
31 characters snuck into the core and we had to manually shorten
it to get the build working.  But it's happened twice in the last
month, most recently with the humanly unswallowable function name
XS_XS__APItest__XSUB_XS_VERSION_undef, so the need for a more
general solution has become not only apparent but mandatory.