exports: rewrite jump table init

The current jump table init fails to initialize a bunch of exported
symbols (forceenv/do_reset/etc...).  Rather than fix just these few
missing pieces, rewrite the code to utilize the existing list of
exported symbols -- _exports.h.  Since every exported symbol has to
be listed in this header, it makes sense to use it so that we only
ever have one list that needs to be updated and things can't fall
out of sync again.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>