Apply RUSTFLAGS env var to rustc builds

Apply RUSTFLAGS env var to rustc builds

This passes RUSTFLAGS to rustc builds for the target architecture.

We don't want to pass the RUSTFLAGS args to multiple architectures because
they may contain architecture-specific flags. Ideally, the scheme
we would use would treat plugins and build scripts - which may not
be for the target architecture - consistently. Unfortunately it's
quite difficult in the current Cargo architecture to seperately
identify build scripts, plugins and their dependencies from
code used by the target.

So the scheme here is very simple:

1) If --target is not specified, RUSTFLAGS applies to all builds.
2) If --target is specified, RUSTFLAGS only applies to builds
   with the Kind::Target target kind, which indicates build units
   derived from the requested --target.

Closes #2112