First, you must setup a cross environment:
setenv TARGET amd64
setenv TARGET_ARCH amd64
setenv MAKEOBJDIRPREFIX /somewhere/obj
cd /usr/src
make kernel-toolchain
At this point, you have two choices:
(1) use the make buildkernel target and build a kernel + modules. This can be streamlined with options like NO_KERNELCLEAN, etc. We use this at work when we’re developing ‘base’ kernel functionality for the arm port we’re undertaking.
(2) use the ‘make buildenv’ functionality. This target puts you in a proper build environment to do things by hand. You need to have done all the steps above, and each time you need to build stuff, you’ll have to use the first 4 commands above followed by ‘make buildenv’. You can then build your module, or whatever else you want, since cc and friends will generate code for amd64.
I use both methods extensively for different types of development. They work great. I have an armenv script:
cd $HOME/p4/imp_arm
setenv TARGET arm
setenv TARGET_ARCH arm
setenv MAKEOBJDIRPREFIX $HOME/obj
make buildenv
which pops me into an environment I can build in. You’ll need to customize it for your environment, clearly, but it should give you the main parts you need to do this sort of thing.
I usually go the other way: build i386 on my fast amd64 box.
Good luck