rp: hook up softfloat rom intrinsics

rp-hal has done this very well already, so we'll just copy their entire
impl again. only div.rs needed some massaging because our sio access
works a little differently, everything else worked as is.

tests/rp/src/bin/float.rs

@@ -0,0 +1,53 @@
+#![no_std]
+#![no_main]
+#![feature(type_alias_impl_trait)]
+
+use defmt::*;
+use embassy_executor::Spawner;
+use embassy_rp::pac;
+use embassy_time::{Duration, Timer};
+use {defmt_rtt as _, panic_probe as _};
+
+#[embassy_executor::main]
+async fn main(_spawner: Spawner) {
+    embassy_rp::init(Default::default());
+    info!("Hello World!");
+
+    const PI_F: f32 = 3.1415926535f32;
+    const PI_D: f64 = 3.14159265358979323846f64;
+
+    unsafe {
+        pac::BUSCTRL
+            .perfsel(0)
+            .write(|r| r.set_perfsel(pac::busctrl::vals::Perfsel::ROM));
+    }
+
+    for i in 0..=360 {
+        let rad_f = (i as f32) * PI_F / 180.0;
+        info!(
+            "{}° float: {=f32} / {=f32} / {=f32} / {=f32}",
+            i,
+            rad_f,
+            rad_f - PI_F,
+            rad_f + PI_F,
+            rad_f % PI_F
+        );
+        let rad_d = (i as f64) * PI_D / 180.0;
+        info!(
+            "{}° double: {=f64} / {=f64} / {=f64} / {=f64}",
+            i,
+            rad_d,
+            rad_d - PI_D,
+            rad_d + PI_D,
+            rad_d % PI_D
+        );
+        Timer::after(Duration::from_millis(10)).await;
+    }
+
+    let rom_accesses = unsafe { pac::BUSCTRL.perfctr(0).read().perfctr() };
+    // every float operation used here uses at least 10 cycles
+    defmt::assert!(rom_accesses >= 360 * 12 * 10);
+
+    info!("Test OK");
+    cortex_m::asm::bkpt();
+}