Export device arm64 rust flags to Bazel

Test: b build //build/bazel/examples/rust:all --config=android
Change-Id: I2656dc963cc4fe0c42da52a0d2a235b5cd86fa13
diff --git a/rust/config/arm64_device.go b/rust/config/arm64_device.go
index 564168b..6c021c7 100644
--- a/rust/config/arm64_device.go
+++ b/rust/config/arm64_device.go
@@ -54,6 +54,7 @@
 			strings.Join(rustFlags, " "))
 	}
 
+	ExportedVars.ExportStringListStaticVariable("DEVICE_ARM64_RUSTC_FLAGS", Arm64RustFlags)
 }
 
 type toolchainArm64 struct {