rust: kbuild: skip gendwarfksyms in bindings.o for Rust >= 1.91.0

Starting with Rust 1.91.0 (released 2025-10-30), in upstream commit
ab91a63d403b ("Ignore intrinsic calls in cross-crate-inlining cost model")
[1][2], `bindings.o` stops containing DWARF debug information because the
`Default` implementations contained `write_bytes()` calls which are now
ignored in that cost model (note that `CLIPPY=1` does not reproduce it).

This means `gendwarfksyms` complains:

      RUSTC L rust/bindings.o
    error: gendwarfksyms: process_module: dwarf_get_units failed: no debugging information?

For the moment, conditionally skip `gendwarfksyms` for Rust >= 1.91.0.

Cc: stable@vger.kernel.org # Needed in 6.12.y and later (Rust is pinned in older LTSs).
Reported-by: Haiyue Wang <haiyuewa@163.com>
Closes: https://lore.kernel.org/rust-for-linux/b8c1c73d-bf8b-4bf2-beb1-84ffdcd60547@163.com/
Link: rust-lang/rust@ab91a63 [1]
Link: rust-lang/rust#145910 [2]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

rust/Makefile

@@ -543,6 +543,7 @@ $(obj)/ffi.o: private skip_gendwarfksyms = 1
 $(obj)/ffi.o: $(src)/ffi.rs $(obj)/compiler_builtins.o FORCE
 	+$(call if_changed_rule,rustc_library)
 
+$(obj)/bindings.o: private skip_gendwarfksyms := $(if $(call rustc-min-version,109100),1)
 $(obj)/bindings.o: private rustc_target_flags = --extern ffi --extern pin_init
 $(obj)/bindings.o: $(src)/bindings/lib.rs \
     $(obj)/ffi.o \