Bypass the load_nif call if mechanism is set to port

When starting erlang from a release using "-mode embedded", the
erlang_nif module is loaded, triggering on_load to call init/0,
regardless of what the mechanism is set to. This would indiscriminately
call load_nif even if mechanism was set to port.

This is not a problem with -mode interactive, because the erlang_nif
module is never loaded by the code server. (Embedded mode loads *all*
modules as part of the release boot process)