Avoid an extra SV when creating $] and $^V

Avoid an extra SV when creating $] and $^V

Originally, GVs always had something in the SV slot.  So, when the
code for $] and $^V started replacing it with another SV, it had to
free the existing SV.

Then commit c69033f2 came along and added the PERL_DONT_CREATE_GVSV
directive.  It necessarily changed a bunch of GvSV()s to GvSVn()s in
gv_fetchpvn_flags.  But it changed these two, even though they didn’t
need it.  So, when PERL_DONT_CREATE_GVSV is true (the default), we
just create and throw away a scalar needlessly.