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.
c2c4c8e146