kbuild: rust: use shared host Rust flags for macros

Proc macros run in the host, so apply those flags for them instead of
just the common ones.

In practice, this currently adds:

    -O -Cstrip=debuginfo -Zallow-features=

and, if enabled, `-Dwarnings` too (i.e. `CONFIG_WERROR`) -- the `macros`
crate should be (and currently is) warning-free, so applying this to it
ensures it is kept that way.

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

Author: ojeda

rust/Makefile

@@ -386,7 +386,7 @@ $(obj)/exports_kernel_generated.h: $(obj)/kernel.o FORCE
 
 quiet_cmd_rustc_procmacro = $(RUSTC_OR_CLIPPY_QUIET) P $@
       cmd_rustc_procmacro = \
-	$(RUSTC_OR_CLIPPY) $(rust_common_flags) \
+	$(RUSTC_OR_CLIPPY) $(KBUILD_HOSTRUSTFLAGS) \
 		-Clinker-flavor=gcc -Clinker=$(HOSTCC) \
 		-Clink-args='$(call escsq,$(KBUILD_HOSTLDFLAGS))' \
 		--emit=dep-info=$(depfile) --emit=link=$@ --extern proc_macro \