add Perl_magic_freemglob() magic vtable method

add Perl_magic_freemglob() magic vtable method

S_mg_free_struct() has a workaround to never free mg->mg_ptr for
PERL_MAGIC_regex_global.

Move this logic into a new magic vtable free method instead, so that
S_mg_free_struct() (which gets called for every type of magic) doesn't
have the overhead of checking every time for mg->mg_type ==
PERL_MAGIC_regex_global.

[ No, I don't know why PERL_MAGIC_regex_global's vtable and methods
  are suffixed mglob rather than regex_global or vice versa ]