* nix/libstore/build.cc (canBuildLocally): Allow building armhf-linux
builds on aarch64-linux.
(DerivationGoal::runChild) Throw error if attempting to build for
armhf-linux on an unsupported platform.
* doc/guix.texi (Invoking guix build): Document how to build natively
for armhf-linux on aarch64-linux. Add note that on some aarch64
machines this is unsupported.