FROMLIST: kbuild: rust: rebuild if the version text changes

Now that `RUSTC_VERSION_TEXT` exists, use it to rebuild `core` when the
version text changes (which in turn will trigger a rebuild of all the
kernel Rust code).

This also applies to proc macros (which only work with the `rustc` that
compiled them), via the already existing dependency on `core.o`. That
is cleaned up in the next commit.

However, this does not cover host programs written in Rust, which is
the same case in the C side.

This is accomplished by referencing directly the generated file, instead
of using the `fixdep` header trick, since we cannot change the Rust
standard library sources. This is not too much of a burden, since it
only needs to be done for `core`.

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Link: https://lore.kernel.org/r/20240808221138.873750-5-ojeda@kernel.org
Change-Id: I9d3ca256f9556ddc4d504db053a8ca3cc13501fa
Signed-off-by: Alice Ryhl <aliceryhl@google.com>

Author: ojeda

rust/Makefile

@@ -370,7 +370,8 @@ $(obj)/core.o: private skip_clippy = 1
 $(obj)/core.o: private skip_flags = -Wunreachable_pub
 $(obj)/core.o: private rustc_objcopy = $(foreach sym,$(redirect-intrinsics),--redefine-sym $(sym)=__rust$(sym))
 $(obj)/core.o: private rustc_target_flags = $(core-cfgs)
-$(obj)/core.o: $(RUST_LIB_SRC)/core/src/lib.rs FORCE
+$(obj)/core.o: $(RUST_LIB_SRC)/core/src/lib.rs \
+    $(wildcard $(objtree)/include/config/RUSTC_VERSION_TEXT) FORCE
 	+$(call if_changed_dep,rustc_library)
 ifdef CONFIG_X86_64
 $(obj)/core.o: scripts/target.json