Allow CORE::GLOBAL:: subs in general to be aliased to consts

Allow CORE::GLOBAL:: subs in general to be aliased to consts

$ perl5.8.8 -e 'use constant foo=>1; BEGIN { *{"CORE::GLOBAL::time"} = \&{"foo"};1} warn time'
1 at -e line 1.
$ perl5.10.1 -e 'use constant foo=>1; BEGIN { *{"CORE::GLOBAL::time"} = \&{"foo"};1} warn time'
1383661397 at -e line 1.

When the storage of constants was optimised in perl 5.10, the code that
checks for global overrides was not updated to account.