Fix RBE by including codegen-backends from rustc/

[acmcarther]

Partially fixes some tests disabled in #111

Resolves (partially) #112 EXCEPT for runfiles.

@xingao267, @mfarrugi

EDIT:
I have to say, this build caching is totally amazing:

See build results latest commit. It didn't have to run any of the tests since the tests from the last commit were still valid as you would expect under local execution.

[xingao267]

The change looks good to me.

Thanks! Happy you like it!

[mfarrugi]

Lgtm, any idea what's up with the runfiles test?

[acmcarther]

Not sure yet:

CI logs from commit 2 of this PR:

Runfiles test stdout:
https://buildkite.com/bazel/rules-rust-rustlang/builds/149#166b2f90-cd68-40d5-9a63-88dbd0cc1215**
Log message from test failure:
https://storage.googleapis.com/bazel-buildkite-artifacts/166b2f90-cd68-40d5-9a63-88dbd0cc1215/xamples/hello_runfiles/hello_runfiles_test/attempt_1.log

Panic site:

rules_rust/examples/hello_runfiles/src/lib.rs

Line 36 in db14152

File::open(runfiles.join("examples/hello_runfiles/data/sample.txt")).unwrap()

rules_rust/examples/hello_runfiles/src/lib.rs

Lines 5 to 17 in db14152

	pub fn get_runfiles_dir() -> io::Result<PathBuf> {
	let mut path = std::env::current_exe()?;
	if cfg!(target_os = "macos") {
	path.pop();
	} else {
	let mut name = path.file_name().unwrap().to_owned();
	name.push(".runfiles");
	path.pop();
	path.push(name);
	}
	Ok(path)

Symptom is test is trying to load the runfile but not finding it. Some historical discussion on runfiles here: #66, describing some inconsistency in treatment among OSes and languages.

If nothing else, some filesystem exploration within the test would probably help figure out where they're being routed to.