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#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>

Makefile

@@ -559,26 +559,31 @@ ifdef building_out_of_srctree
 	{ echo "# this is build directory, ignore it"; echo "*"; } > .gitignore
 endif
 
-# The expansion should be delayed until arch/$(SRCARCH)/Makefile is included.
-# Some architectures define CROSS_COMPILE in arch/$(SRCARCH)/Makefile.
-# CC_VERSION_TEXT is referenced from Kconfig (so it needs export),
-# and from include/config/auto.conf.cmd to detect the compiler upgrade.
-CC_VERSION_TEXT = $(shell $(CC) --version 2>/dev/null | head -n 1 | sed 's/\#//g')
+TENTATIVE_CLANG_FLAGS := -Werror=unknown-warning-option
 
-ifneq ($(findstring clang,$(CC_VERSION_TEXT)),)
 ifneq ($(CROSS_COMPILE),)
-CLANG_FLAGS	+= --target=$(notdir $(CROSS_COMPILE:%-=%))
+TENTATIVE_CLANG_FLAGS	+= --target=$(notdir $(CROSS_COMPILE:%-=%))
 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))
 GCC_TOOLCHAIN	:= $(realpath $(GCC_TOOLCHAIN_DIR)/..)
 endif
 ifneq ($(GCC_TOOLCHAIN),)
-CLANG_FLAGS	+= --gcc-toolchain=$(GCC_TOOLCHAIN)
+TENTATIVE_CLANG_FLAGS	+= --gcc-toolchain=$(GCC_TOOLCHAIN)
 endif
 ifneq ($(LLVM_IAS),1)
-CLANG_FLAGS	+= -no-integrated-as
+TENTATIVE_CLANG_FLAGS	+= -no-integrated-as
 endif
-CLANG_FLAGS	+= -Werror=unknown-warning-option
+
+export TENTATIVE_CLANG_FLAGS
+
+# The expansion should be delayed until arch/$(SRCARCH)/Makefile is included.
+# Some architectures define CROSS_COMPILE in arch/$(SRCARCH)/Makefile.
+# CC_VERSION_TEXT is referenced from Kconfig (so it needs export),
+# and from include/config/auto.conf.cmd to detect the compiler upgrade.
+CC_VERSION_TEXT = $(shell $(CC) --version 2>/dev/null | head -n 1 | sed 's/\#//g')
+
+ifneq ($(findstring clang,$(CC_VERSION_TEXT)),)
+CLANG_FLAGS	+= $(TENTATIVE_CLANG_FLAGS)
 KBUILD_CFLAGS	+= $(CLANG_FLAGS)
 KBUILD_AFLAGS	+= $(CLANG_FLAGS)
 export CLANG_FLAGS