arm64: rust: clean Rust 1.85.0 warning using softfloat target

commit 446a835 upstream.

Starting with Rust 1.85.0 (to be released 2025-02-20), `rustc` warns
[1] about disabling neon in the aarch64 hardfloat target:

    warning: target feature `neon` cannot be toggled with
             `-Ctarget-feature`: unsound on hard-float targets
             because it changes float ABI
      |
      = note: this was previously accepted by the compiler but
              is being phased out; it will become a hard error
              in a future release!
      = note: for more information, see issue #116344
              <rust-lang/rust#116344>

Thus, instead, use the softfloat target instead.

While trying it out, I found that the kernel sanitizers were not enabled
for that built-in target [2]. Upstream Rust agreed to backport
the enablement for the current beta so that it is ready for
the 1.85.0 release [3] -- thanks!

However, that still means that before Rust 1.85.0, we cannot switch
since sanitizers could be in use. Thus conditionally do so.

Cc: stable@vger.kernel.org # Needed in 6.12.y and 6.13.y only (Rust is pinned in older LTSs).
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Matthew Maurer <mmaurer@google.com>
Cc: Alice Ryhl <aliceryhl@google.com>
Cc: Ralf Jung <post@ralfj.de>
Cc: Jubilee Young <workingjubilee@gmail.com>
Link: rust-lang/rust#133417 [1]
Link: https://rust-lang.zulipchat.com/#narrow/channel/131828-t-compiler/topic/arm64.20neon.20.60-Ctarget-feature.60.20warning/near/495358442 [2]
Link: rust-lang/rust#135905 [3]
Link: rust-lang/rust#116344
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Reviewed-by: Trevor Gross <tmgross@umich.edu>
Tested-by: Matthew Maurer <mmaurer@google.com>
Reviewed-by: Ralf Jung <post@ralfj.de>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20250210163732.281786-1-ojeda@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: ojeda

arch/arm64/Makefile

@@ -48,7 +48,11 @@ KBUILD_CFLAGS	+= $(CC_FLAGS_NO_FPU) \
 KBUILD_CFLAGS	+= $(call cc-disable-warning, psabi)
 KBUILD_AFLAGS	+= $(compat_vdso)
 
+ifeq ($(call test-ge, $(CONFIG_RUSTC_VERSION), 108500),y)
+KBUILD_RUSTFLAGS += --target=aarch64-unknown-none-softfloat
+else
 KBUILD_RUSTFLAGS += --target=aarch64-unknown-none -Ctarget-feature="-neon"
+endif
 
 KBUILD_CFLAGS	+= $(call cc-option,-mabi=lp64)
 KBUILD_AFLAGS	+= $(call cc-option,-mabi=lp64)