This allows specifying additional systems that a machine is able to
build for. This may apply on some armv7-capable aarch64 processors, or
on systems using qemu-user with binfmt-misc to support transparent
execution of foreign-arch programs.

This removes the previous hard-coded assumptions about which systems are
ABI-compatible with which other systems, and instead relies on the user
to specify any additional platforms that they have ensured compatibility
for and wish to build for locally.

NixOS should probably add i686-linux on x86_64-linux systems for this
setting by default.

This allows building armv[67]l-linux derivations on compatible aarch64
machines. Failure to add the architecture may result from missing
hardware support, in which case we can't run 32-bit binaries and don't
need to restrict them with seccomp anyway,

also document it

Support extra compatible architectures (#1916)