Chez for architectures without native backends

[LiberalArtist]

@LiberalArtist

I've gotten a little further with this, after figuring out an issue with my Guix recipe, but now the Chez build process is trying to run my cross-compiled scheme as part of the build process.

[...]

Since tpb64l/bin/tpb64l/scheme was compiled for powerpc64le-linux-gnu, it doesn't run on x86_64-linux-gnu.

I'm not sure how to deal with this in general for cross-compiling Chez, but it seems especially likely for things to get confused when trying to cross-compile a pb variant that is also supported by the build machine. It seems like nothing in my configure flags communicates that this is supposed to be a cross build, and I'm not seeing any way to add that information, either.

Reading the BUILDING file again, I found that I could use make kernel when cross-compiling to "compile just the C sources to produce the executable so that running can use existing bootfiles." This seems to have worked! I think there's room for improvement to make things go more smoothly, but I think this should be enough to let Guix start distributing Racket's Chez Scheme for all of Guix's supported architectures with the Racket 8.6 release.