Clarify dependency requirements in the face of cross-compilation

Clarify dependency requirements in the face of cross-compilation

The requirement on dependencies was phrased in terms of "host tools",
but it was also intended to apply equally to targets that only support
cross-compilation. Only the exception (for libraries commonly needed for
binaries on the target) was intended to apply to host tools. Reword the
requirement to talk about the dependencies required for "compiling,
linking,and emitting functional binaries, libraries, or other code for
the target", rather than generically in terms of dependencies for
rustc/cargo.

This doesn't change the net effect of the requirements, since other
requirements already stated that the target can't make the Rust
toolchain depend on proprietary libraries. However, this should make the
requirements clearer.