Add empty test binary for riscv

tests/riscv32/src/bin/empty.rs

@@ -0,0 +1,16 @@
+#![no_std]
+#![no_main]
+#![feature(type_alias_impl_trait)]
+
+use embassy_executor::Spawner;
+
+#[panic_handler]
+fn panic (_info: &core::panic::PanicInfo) -> ! {
+    loop {}
+}
+
+#[embassy_executor::main]
+async fn main(_spawner: Spawner) {
+    // Don't do anything, just make sure it compiles.
+    loop {}
+}