ANDROID: Define location of Rust tools and toolchain

This is used by the build system to build kernels with
Rust support.

Bug: 270672379
Test: tools/bazel run //common-modules/virtual-device:virtual_device_x86_64_dist
    CONFIG_RUST_IS_AVAILABLE=y set in .config
Change-Id: Id8124b12fe2b1238838235a21ea824230c7621f4
Signed-off-by: Jeff Vander Stoep <jeffv@google.com>

Author: jeffvanderstoep

build.config.constants

@@ -1,4 +1,7 @@
 BRANCH=android-mainline
 CLANG_VERSION=r475365b
+RUSTC_VERSION=1.68.0
+CLANGTOOLS_PREBUILT_BIN=prebuilts/clang-tools/linux-x86/bin
+RUST_PREBUILT_BIN=prebuilts/rust/linux-x86/${RUSTC_VERSION}/bin
 AARCH64_NDK_TRIPLE=aarch64-linux-android31
 X86_64_NDK_TRIPLE=x86_64-linux-android31