fix: Add (even more) #[avr_skip] for floats

Tale as old as the world - there's an ABI mismatch:
https://github.com/rust-lang/compiler-builtins/pull/527

Fortunately, newest GCCs (from v11, it seems) actually provide most of
those intrinsics (even for f64!), so that's pretty cool.

(the only intrinsics not provided by GCC are `__powisf2` & `__powidf2`,
but our codegen for AVR doesn't emit those anyway.)

Fixes https://github.com/rust-lang/rust/issues/118079.

library/compiler-builtins/src/float/mul.rs

@@ -185,12 +185,14 @@ where
 }
 
 intrinsics! {
+    #[avr_skip]
     #[aapcs_on_arm]
     #[arm_aeabi_alias = __aeabi_fmul]
     pub extern "C" fn __mulsf3(a: f32, b: f32) -> f32 {
         mul(a, b)
     }
 
+    #[avr_skip]
     #[aapcs_on_arm]
     #[arm_aeabi_alias = __aeabi_dmul]
     pub extern "C" fn __muldf3(a: f64, b: f64) -> f64 {