rust: Add rust_toolchain_library module.

Adds a new rust_toolchain_library module, a module which can compute the
source path from the toolchain root.

Bug: 140642453
Test: m

Change-Id: I2d90722880279533eb93d2d946ac8c31fc0b1835
diff --git a/rust/Android.bp b/rust/Android.bp
index cda2dbc..5e14da8 100644
--- a/rust/Android.bp
+++ b/rust/Android.bp
@@ -38,6 +38,7 @@
         "strip.go",
         "test.go",
         "testing.go",
+        "toolchain_library.go",
     ],
     testSrcs: [
         "benchmark_test.go",