Assigning to DEFSV leaks if PL_defgv's gp_sv isn't set.

Assigning to DEFSV leaks if PL_defgv's gp_sv isn't set.
As Nicholas already noted in a FIXME, assigning to DEFSV should
use GvSV instead of GvSVn. This change ensures that, at least
under -DPERL_CORE, DEFSV cannot be assigned to and introduces
a DEFSV_set macro to allow setting DEFSV.
This fixes #53038: map leaks memory.

p4raw-id: //depot/perl@34776