For PERL_CORE, don't undefine HAS_QUAD when building with 32 bit IVs.

For PERL_CORE, don't undefine HAS_QUAD when building with 32 bit IVs.

When commit de1c2614758a00c0 added support for $Config{d_quad} and HAS_QUAD
in Oct 1999, it also undefined HAS_QUAD. It's been that way ever since.

This isn't actually correct on a platform with a 64 bit type available, but
changing it now will cause too much breakage on CPAN for too little gain
(See RT #119753). However, we do need HAS_QUAD in the core for use by the
drand48 code, so make the undef conditional on PERL_CORE not being set.