Make defined(${'$'}) return true

Commit 0e21945 made $$ into a magical variable that is not created
on startup.

Usually perl pretends that built-in vars created on the fly have
always existed.  But commit 0e21945 did not add $$ to the list of
such variables in is_gv_magical_sv.  So defined ${'$'} started return-
ing false.

Author: Father Chrysostomos

gv.c

@@ -2771,6 +2771,7 @@ Perl_is_gv_magical_sv(pTHX_ SV *const name_sv, U32 flags)
 	case '>':
 	case '\\':
 	case '/':
+	case '$':
 	case '|':
 	case '+':
 	case ';':