Sorry, I think this needs more work. This is able to cross-build a native go compiler, but breaks when trying to build a cross-compiler.

Ok, this now works for cross compiling go packages and cross building the go compiler, but is still very ugly ;)
I suspect the HostTarget/TargetTarget thing is a bit of a hack, to trick go's monolithic bootstrap script into building for the proper host/target. However this is working for me and is very useful since go won't bootstrap natively on ppc64le (not a supported go 1.4 target) so thought I'd share.

I don't think you should need that conditional definition of targetCC. when host == target, pkgsHostTarget should also equal pkgsTargetTarget.

Remember that stdenv (confusingly) has to tools to build the stage, not the tools built in that stage, so pkgsTargetTarget.stdenv.cc in fact would be something like pkgsHostTarget.{gcc.clang}.

Hmm, well I agree it doesn't make sense, but without the targetcc hack, pkgsCross.powernv.go fails to build with

Building Go cmd/dist using /nix/store/5z4x1875l94bb66jk9zin41am0664xw0-go-bootstrap/share/go. ()
go tool dist: cannot invoke C compiler "/nix/store/ixanm098dmc7nyxmnp7jb3gcypcsm816-gcc-debug-wrapper-9.3.0/bin/cc": exit status 1

Go needs a system C compiler for use with cgo.
To set a C compiler, set CC=the-compiler.
To disable cgo, set CGO_ENABLED=0.

Command output:

/nix/store/ixanm098dmc7nyxmnp7jb3gcypcsm816-gcc-debug-wrapper-9.3.0/bin/cc: line 195: /nix/store/mvn4chpqxmaab4ccldinjyis1nffrpwz-gcc-debug-9.3.0-powerpc64le-unknown-linux-gnu/bin/gcc: cannot execute binary file: Exec format error

Ah, I see the problem: that compiler isn't runnable at build time.

You want pkgsBuildTarget.targetPackages.stdenv.cc; the idea is that the targetPackages and stdenv cancel out, so one is left with something like pkgsBuildTarget.clang, as desired.

Thank you so much, that is working beautifully now. I've kept the targetCC variable, let me know if you have a better name for it.