platforms: add aarch64

This lets us test our new CQ coverage, and have a platform we'll
actually need. Whee.

BUG=None
TEST=bazel-5 build --platforms=@rules_cros//cros/platforms:cros_aarch64

Change-Id: I1bb26f986895fd0c9151a0e7414f71d4853b1cfb
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/rules_cros/+/3595033
Reviewed-by: Chris McDonald <cjmcdonald@chromium.org>
Commit-Queue: George Burgess <gbiv@chromium.org>
Tested-by: George Burgess <gbiv@chromium.org>
diff --git a/cros/toolchain/platforms.bzl b/cros/toolchain/platforms.bzl
index 07f417e..4d8d4ec 100644
--- a/cros/toolchain/platforms.bzl
+++ b/cros/toolchain/platforms.bzl
@@ -49,4 +49,11 @@
         vendor = "cros",
         abi = "gnueabihf",
     ),
+
+    # Target aarch64 toolchain
+    ToolchainDesc(
+        cpu_arch = "aarch64",
+        vendor = "cros",
+        abi = "gnu",
+    ),
 ]