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/.cargo/config.toml

@@ -1,6 +1,8 @@
 [unstable]
-build-std = ["core"]
-build-std-features = ["panic_immediate_abort"]
+# enabling these breaks the float tests during linking, with intrinsics
+# duplicated between embassy-rp and compilter_builtins
+#build-std = ["core"]
+#build-std-features = ["panic_immediate_abort"]
 
 [target.'cfg(all(target_arch = "arm", target_os = "none"))']
 #runner = "teleprobe client run --target rpi-pico --elf"