op.c:ck_glob: Check PL_globhook before loading File::Glob

op.c:ck_glob: Check PL_globhook before loading File::Glob

By loading File::Glob when there is no CORE::GLOBAL::glob, we just end
up calling Perl_load_module for every glob op, since File::Glob no
longer uses CORE::GLOBAL::glob by default.

We could just as well check whether PL_globhook is set, which would
be faster.

(File::Glob sets PL_globhook when it loads.  In 5.14, it didn’t
set anything, but ck_glob itself would set CORE::GLOBAL::glob to
File::Glob::csh_glob.)