Some native library sets, such as components of libc, libgcc, or
compiler-builtins, require backwards or circular references.
Additionally, because link order doesn't matter for native libraries
when using dynamic linking, many crates don't take it into account for
static linking either; this can introduce platform-specific issues.
Rather than make the order of `#[link]` lines or `cargo:rustc-link-lib`
lines semantically significant (and potentially even require specifying
some libraries twice), wrap each set of libraries in linker groups. This
ensures that symbols in all libraries resolve correctly regardless of
library order.

Linker groups of reasonable sizes don't have a substantial cost, but
putting the entirey of a large link line inside a linker group can
add enough cost to be noticeable. To minimize the cost of linker groups,
use separate groups for local native libraries, upstream crates, and
upstream native libraries. This doesn't allow backreferences between
those groups, but in most cases we won't want such backreferences.

These fields were both only used for library ordering, so they're now
both unused.