rust: add intrinsics to fix -Os builds

Alice reported [1] that an arm64 build failed with:

    ld.lld: error: undefined symbol: __extendsfdf2
    >>> referenced by core.a6f5fc5794e7b7b3-cgu.0
    >>>               rust/core.o:(<f32>::midpoint) in archive vmlinux.a
    >>> referenced by core.a6f5fc5794e7b7b3-cgu.0
    >>>               rust/core.o:(<f32>::midpoint) in archive vmlinux.a

    ld.lld: error: undefined symbol: __truncdfsf2
    >>> referenced by core.a6f5fc5794e7b7b3-cgu.0
    >>>               rust/core.o:(<f32>::midpoint) in archive vmlinux.a

Rust 1.80.0 or later together with `CONFIG_CC_OPTIMIZE_FOR_SIZE=y`
is what triggers it.

In addition, x86_64 builds also fail the same way.

Similarly, compiling with Rust 1.82.0 (currently in nightly) makes
another one appear, possibly due to the LLVM 19 upgrade there:

    ld.lld: error: undefined symbol: __eqdf2
    >>> referenced by core.20495ea57a9f069d-cgu.0
    >>>               rust/core.o:(<f64>::next_up) in archive vmlinux.a
    >>> referenced by core.20495ea57a9f069d-cgu.0
    >>>               rust/core.o:(<f64>::next_down) in archive vmlinux.a

Gary adds [1]:

> Usually the fix on rustc side is to mark those functions as `#[inline]`
>
> All of {midpoint,next_up,next_down} are indeed unstable functions not
> marked as inline...

Fix all those by adding those intrinsics to our usual workaround.

Cc: Gary Guo <gary@garyguo.net>
Reported-by: Alice Ryhl <aliceryhl@google.com>
Closes: https://rust-for-linux.zulipchat.com/#narrow/stream/288089-General/topic/v6.2E11-rc1.20doesn't.20build.20for.20arm64/near/455637364
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Tested-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20240806150619.192882-1-ojeda@kernel.org

rust/Makefile

@@ -354,8 +354,8 @@ rust-analyzer:
 		$(if $(KBUILD_EXTMOD),$(extmod_prefix),$(objtree))/rust-project.json
 
 redirect-intrinsics = \
-	__addsf3 __eqsf2 __gesf2 __lesf2 __ltsf2 __mulsf3 __nesf2 __unordsf2 \
-	__adddf3 __ledf2 __ltdf2 __muldf3 __unorddf2 \
+	__addsf3 __eqsf2 __extendsfdf2 __gesf2 __lesf2 __ltsf2 __mulsf3 __nesf2 __truncdfsf2 __unordsf2 \
+	__adddf3 __eqdf2 __ledf2 __ltdf2 __muldf3 __unorddf2 \
 	__muloti4 __multi3 \
 	__udivmodti4 __udivti3 __umodti3
 

rust/compiler_builtins.rs

@@ -40,16 +40,19 @@ macro_rules! define_panicking_intrinsics(
 define_panicking_intrinsics!("`f32` should not be used", {
     __addsf3,
     __eqsf2,
+    __extendsfdf2,
     __gesf2,
     __lesf2,
     __ltsf2,
     __mulsf3,
     __nesf2,
+    __truncdfsf2,
     __unordsf2,
 });
 
 define_panicking_intrinsics!("`f64` should not be used", {
     __adddf3,
+    __eqdf2,
     __ledf2,
     __ltdf2,
     __muldf3,