refactor(arch): split the dummy arch into multiple submodule files

Split the `dummy` arch to make it more manageable when it keeps growing.

src/riot-rs-embassy/src/arch/dummy/executor.rs

@@ -0,0 +1,32 @@
+use embassy_executor::SpawnToken;
+
+use crate::arch;
+
+pub struct Executor;
+
+impl Executor {
+    #[allow(clippy::new_without_default)]
+    pub const fn new() -> Self {
+        // Actually return a value instead of marking it unimplemented like other dummy
+        // functions, because this function is const and is thus run during compilation
+        Self {}
+    }
+
+    pub fn start(&self, _: arch::SWI) {
+        unimplemented!();
+    }
+
+    pub fn spawner(&self) -> Spawner {
+        unimplemented!();
+    }
+}
+
+pub struct Spawner;
+
+impl Spawner {
+    #[allow(clippy::result_unit_err)]
+    pub fn spawn<S>(&self, _token: SpawnToken<S>) -> Result<(), ()> {
+        unimplemented!();
+    }
+    pub fn must_spawn<S>(&self, _token: SpawnToken<S>) {}
+}

src/riot-rs-embassy/src/arch/dummy/hwrng.rs

@@ -0,0 +1,5 @@
+use crate::arch;
+
+pub fn construct_rng(_peripherals: &mut arch::OptionalPeripherals) {
+    unimplemented!();
+}

src/riot-rs-embassy/src/arch/dummy/mod.rs

@@ -0,0 +1,34 @@
+//! Dummy module used to satisfy platform-independent tooling.
+
+mod executor;
+
+#[cfg(feature = "hwrng")]
+pub mod hwrng;
+
+#[cfg(feature = "usb")]
+pub mod usb;
+
+pub use executor::{Executor, Spawner};
+
+/// Dummy type.
+///
+/// See the `OptionalPeripherals` type of your Embassy architecture crate instead.
+pub struct OptionalPeripherals;
+
+/// Dummy type.
+pub struct Peripherals;
+
+impl From<Peripherals> for OptionalPeripherals {
+    fn from(_peripherals: Peripherals) -> Self {
+        Self {}
+    }
+}
+
+#[derive(Default)]
+pub struct Config;
+
+pub fn init(_config: Config) -> OptionalPeripherals {
+    unimplemented!();
+}
+
+pub struct SWI;