FROMLIST: kbuild: rust: replace proc macros dependency on core.o with the version text

With the `RUSTC_VERSION_TEXT` rebuild support in place, now proc macros
can depend on that instead of `core.o`.

This means that both the `core` and `macros` crates can be built in
parallel, and that touching `core.o` does not trigger a rebuild of the
proc macros.

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

Link: https://lore.kernel.org/r/20240808221138.873750-6-ojeda@kernel.org
Change-Id: I9c5c16cc4fb826c4f70823b36f9a47a1197fd47c
Signed-off-by: Alice Ryhl <aliceryhl@google.com>

Author: ojeda

rust/Makefile

@@ -329,9 +329,8 @@ quiet_cmd_rustc_procmacro = $(RUSTC_OR_CLIPPY_QUIET) P $@
 		--crate-name $(patsubst lib%.so,%,$(notdir $@)) $<
 
 # Procedural macros can only be used with the `rustc` that compiled it.
-# Therefore, to get `libmacros.so` automatically recompiled when the compiler
-# version changes, we add `core.o` as a dependency (even if it is not needed).
-$(obj)/libmacros.so: $(src)/macros/lib.rs $(obj)/core.o FORCE
+$(obj)/libmacros.so: $(src)/macros/lib.rs \
+    $(wildcard $(objtree)/include/config/RUSTC_VERSION_TEXT) FORCE
 	+$(call if_changed_dep,rustc_procmacro)
 
 quiet_cmd_rustc_library = $(if $(skip_clippy),RUSTC,$(RUSTC_OR_CLIPPY_QUIET)) L $@