Add target definition needed for Rust 1.83 update

The new file group rule is needed for the Rust 1.83 update.  Because
this rule needs to also work with the current version of Rust (1.82) we
add a set of empty files for Soong to find.

Test: m libcompiler_builtins.rust_sysroot
Bug: 384401387
Change-Id: I182c2455795f327be5609760183fb785aee73ee0
5 files changed