Support overriding build scripts for Rust crates

Some crates provide a way to override their build script
that would otherwise at least locate a needed shared
library. This change provides a way for GN based Rust
binaries to make this happen by using a “native_libs”
attribute.

Change-Id: I3d5b549fd80cc58a7308cfc6835cd6c72709f5dd
3 files changed