Eliminate is_gv_magical_sv

Eliminate is_gv_magical_sv

This resolves perl bug #97978.

Many built-in variables, like $], are actually created on the fly
when first accessed.  Perl likes to pretend that these variables have
always existed, so it autovivifies the *] glob even in rvalue context
(e.g., defined *{"]"}, close "]").

The list of variables that were autovivified was maintained separ-
ately (in is_gv_magical_sv) from the code that actually creates
them (gv_fetchpvn_flags).  ‘Maintained’ is not actually precise: it
*wasn’t* being maintained, and there were new variables that never
got added to is_gv_magical_sv and one deleted variable that was
never removed.

There are only two pieces of code that call is_gv_magical_sv, both in
pp.c: S_rv2gv (called by *{} and also the implicit *{} that functions
like close() provide) and Perl_softrefxv (called by ${}, @{}, %{}).

In both cases, the glob is immediately autovivified if
is_gv_magical_sv returns true.

So this commit eliminates the extra maintenance burden by extirpat-
ing is_gv_magical_sv altogether, and replacing it with a new flag to
gv_fetchpvn_flags, GvADDMG, which will autovivify a glob *if* it’s a
magical one.

It does make defined(*{"frobbly"}) slightly slower, in that it creates
a temporary glob and then frees it when it sees nothing magical has
been done with it.  But this case is rare enough it should not matter.
At least I got rid of the bugginess.