Makefile: generate CLANG_FLAGS even in GCC builds

To support Rust under GCC-built kernels, we need to save the flags that
would have been passed if the kernel was being compiled with Clang.

The reason is that `bindgen` -- the tool we use to generate Rust
bindings to the C side of the kernel -- relies on `libclang` to
parse C. Ideally:

  - `bindgen` would support a GCC backend (requested at [1]),

  - or the Clang driver would be perfectly compatible with GCC,
    including plugins. Unlikely, of course, but perhaps a big
    subset of configs may be possible to guarantee to be kept
    compatible nevertheless.

This is also the reason why GCC builds are very experimental and some
configurations may not work (e.g. `GCC_PLUGIN_RANDSTRUCT`). However,
we keep GCC builds working (for some example configs) in the CI
to avoid diverging/regressing further, so that we are better prepared
for the future when a solution might become available.

[1] rust-lang/rust-bindgen#1949

Link: Rust-for-Linux/linux#167
Co-developed-by: Alex Gaynor <alex.gaynor@gmail.com>
Signed-off-by: Alex Gaynor <alex.gaynor@gmail.com>
Co-developed-by: Geoffrey Thomas <geofft@ldpreload.com>
Signed-off-by: Geoffrey Thomas <geofft@ldpreload.com>
Co-developed-by: Finn Behrens <me@kloenk.de>
Signed-off-by: Finn Behrens <me@kloenk.de>
Co-developed-by: Adam Bratschi-Kaye <ark.email@gmail.com>
Signed-off-by: Adam Bratschi-Kaye <ark.email@gmail.com>
Co-developed-by: Wedson Almeida Filho <wedsonaf@google.com>
Signed-off-by: Wedson Almeida Filho <wedsonaf@google.com>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

Makefile

@@ -583,18 +583,23 @@ endif
 # and from include/config/auto.conf.cmd to detect the compiler upgrade.
 CC_VERSION_TEXT = $(subst $(pound),,$(shell $(CC) --version 2>/dev/null | head -n 1))
 
-ifneq ($(findstring clang,$(CC_VERSION_TEXT)),)
+TENTATIVE_CLANG_FLAGS := -Werror=unknown-warning-option
+
 ifneq ($(CROSS_COMPILE),)
-CLANG_FLAGS	+= --target=$(notdir $(CROSS_COMPILE:%-=%))
+TENTATIVE_CLANG_FLAGS	+= --target=$(notdir $(CROSS_COMPILE:%-=%))
 endif
 ifeq ($(LLVM_IAS),1)
-CLANG_FLAGS	+= -integrated-as
+TENTATIVE_CLANG_FLAGS	+= -integrated-as
 else
-CLANG_FLAGS	+= -no-integrated-as
+TENTATIVE_CLANG_FLAGS	+= -no-integrated-as
 GCC_TOOLCHAIN_DIR := $(dir $(shell which $(CROSS_COMPILE)elfedit))
-CLANG_FLAGS	+= --prefix=$(GCC_TOOLCHAIN_DIR)$(notdir $(CROSS_COMPILE))
+TENTATIVE_CLANG_FLAGS	+= --prefix=$(GCC_TOOLCHAIN_DIR)$(notdir $(CROSS_COMPILE))
 endif
-CLANG_FLAGS	+= -Werror=unknown-warning-option
+
+export TENTATIVE_CLANG_FLAGS
+
+ifneq ($(findstring clang,$(CC_VERSION_TEXT)),)
+CLANG_FLAGS	+= $(TENTATIVE_CLANG_FLAGS)
 KBUILD_CFLAGS	+= $(CLANG_FLAGS)
 KBUILD_AFLAGS	+= $(CLANG_FLAGS)
 export CLANG_FLAGS