Backport XS_(IN|EX)TERNAL, be explicit about linkage

Backport XS_(IN|EX)TERNAL, be explicit about linkage

With XS(name) defaulting to exporting symbols again since the
previous commit, ExtUtils::ParseXS will now instead use explicit
XS_EXTERNAL/XS_INTERNAL in its place. This allows backporting
of the linkage changes to perls as old as 5.10.0 (and possibly
further).