stand: ensure that the efi directory's dependencies are correct

efi, like the various ${MACHINE} directories, should have a dependency on
the enabled interpreters.

The general rule here is that any top-level directory that has a program at
any depth within that includes loader.mk should add ${INTERP_DEPENDS} added
to its dependencies so that the appropriate ficl/lua bits are ready before
they begin.

Note that the only directories in-tree that require it but will not get it
in a more appropriate manner are i386 (on amd64), efi, and userboot. i386
and userboot are handled explicitly in Makefile.amd64 where they are added
to S.yes.

Reported-by: bcran
(cherry picked from commit 7012461c9bf6375cd0b14de16b3b4a753c5c1c7a)