rust: allow to use INIT_STACK_ALL_ZERO

With CONFIG_INIT_STACK_ALL_ZERO enabled, bindgen passes
-ftrivial-auto-var-init=zero to clang, that triggers the following
error:

 error: '-ftrivial-auto-var-init=zero' hasn't been enabled; enable it at your own peril for benchmarking purpose only with '-enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang'

However, this additional option that is currently required by clang is
deprecated since clang-16 and going to be removed in the future,
likely with clang-18.

So, make sure bindgen is using this extra option if the major version of
the libclang used by bindgen is < 16.

In this way we can enable CONFIG_INIT_STACK_ALL_ZERO with CONFIG_RUST
without triggering any build error.

Link: https://github.com/llvm/llvm-project/issues/44842
Link: https://github.com/llvm/llvm-project/blob/llvmorg-16.0.0-rc2/clang/docs/ReleaseNotes.rst#deprecated-compiler-flagsSigned-off-by: Andrea Righi <andrea.righi@canonical.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
[Changed to < 16, added link and reworded]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/Makefile

@@ -262,6 +262,20 @@ BINDGEN_TARGET		:= $(BINDGEN_TARGET_$(SRCARCH))
 # some configurations, with new GCC versions, etc.
 bindgen_extra_c_flags = -w --target=$(BINDGEN_TARGET)
 
+# Auto variable zero-initialization requires an additional special option with
+# clang that is going to be removed sometime in the future (likely in
+# clang-18), so make sure to pass this option only if clang supports it
+# (libclang major version < 16).
+#
+# https://github.com/llvm/llvm-project/issues/44842
+# https://github.com/llvm/llvm-project/blob/llvmorg-16.0.0-rc2/clang/docs/ReleaseNotes.rst#deprecated-compiler-flags
+ifdef CONFIG_INIT_STACK_ALL_ZERO
+libclang_maj_ver=$(shell $(BINDGEN) $(srctree)/scripts/rust_is_available_bindgen_libclang.h 2>&1 | sed -ne 's/.*clang version \([0-9]*\).*/\1/p')
+ifeq ($(shell expr $(libclang_maj_ver) \< 16), 1)
+bindgen_extra_c_flags += -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang
+endif
+endif
+
 bindgen_c_flags = $(filter-out $(bindgen_skip_c_flags), $(c_flags)) \
 	$(bindgen_extra_c_flags)
 endif