rust: Force all dependencies to be linked, used or not

`rustc` added an unstable feature that makes `--extern` do what we
originally wanted - it links the provided library, *even* if the source
code does not reference it.

The primary use case for this is that it makes it possible for us to
build dylibs out of `#![no_std]` crates without modifying the source,
because we can pass in `--extern force:path/to/libstd.dylib.so`.

Bug: 168927099
Test: m, TH
Change-Id: Ibc3b75012d54b73e40ae609784cdc74e02e8f008