Makefile: Fix GCC_TOOLCHAIN_DIR prefix for Clang cross compilation

* Fix '--prefix=' value for newer versions of clang

Newer versions of clang only look for $(COMPAT_GCC_TOOLCHAIN_DIR)as [1],
rather than $(COMPAT_GCC_TOOLCHAIN_DIR)$(CROSS_COMPILE_COMPAT)as,
resulting in the following build error:

$ make -skj"$(nproc)" ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- \
CROSS_COMPILE_COMPAT=arm-linux-gnueabi- LLVM=1 O=out/aarch64 distclean \
defconfig arch/arm64/kernel/vdso32/
...
/home/nathan/cbl/toolchains/llvm-binutils/bin/as: unrecognized option '-EL'
clang-12: error: assembler command failed with exit code 1 (use -v to see invocation)
make[3]: *** [arch/arm64/kernel/vdso32/Makefile:181: arch/arm64/kernel/vdso32/note.o] Error 1
...

Adding the value of CROSS_COMPILE_COMPAT (adding notdir to account for a
full path for CROSS_COMPILE_COMPAT) fixes this issue, which matches the
solution done for the main Makefile [2].

When CROSS_COMPILE is set (e.g. aarch64-linux-gnu-), if
$(CROSS_COMPILE)elfedit is found at /usr/bin/aarch64-linux-gnu-elfedit,
GCC_TOOLCHAIN_DIR will be set to /usr/bin/.  --prefix= will be set to
/usr/bin/ and Clang as of 11 will search for both
$(prefix)aarch64-linux-gnu-$needle and $(prefix)$needle.

GCC searchs for $(prefix)aarch64-linux-gnu/$version/$needle,
$(prefix)aarch64-linux-gnu/$needle and $(prefix)$needle. In practice,
$(prefix)aarch64-linux-gnu/$needle rarely contains executables.

To better model how GCC's -B/--prefix takes in effect in practice, newer
Clang (since
llvm/llvm-project@3452a0d)
only searches for $(prefix)$needle. Currently it will find /usr/bin/as
instead of /usr/bin/aarch64-linux-gnu-as.

Set --prefix= to $(GCC_TOOLCHAIN_DIR)$(notdir $(CROSS_COMPILE))
(/usr/bin/aarch64-linux-gnu-) so that newer Clang can find the
appropriate cross compiling GNU as (when -no-integrated-as is in
effect).

Cc: stable@vger.kernel.org
Reported-by: Nathan Chancellor <natechancellor@gmail.com>
Change-Id: I7e1a9d21fa53b46eda9c6ff4c413aca59f91ceeb
Signed-off-by: Fangrui Song <maskray@google.com>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Tested-by: Nathan Chancellor <natechancellor@gmail.com>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Link: ClangBuiltLinux/linux#1099
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Danny Lin <danny@kdrag0n.dev>
Signed-off-by: Aoihara <veronicadisa6@gmail.com>
Signed-off-by: danascape <saalim.priv@gmail.com>
Signed-off-by: Kneba <abenkenary3@gmail.com>

Makefile

@@ -645,7 +645,6 @@ endif
 ifneq ($(GCC_TOOLCHAIN),)
 CLANG_FLAGS	+= --gcc-toolchain=$(GCC_TOOLCHAIN)
 endif
-CLANG_FLAGS	+= -no-integrated-as
 CLANG_FLAGS	+= -Werror=unknown-warning-option
 CLANG_FLAGS     += -Wno-compound-token-split-by-space
 CLANG_FLAGS    += $(call cc-option, -Wno-misleading-indentation)
@@ -782,6 +781,9 @@ KBUILD_CFLAGS += $(call cc-disable-warning, duplicate-decl-specifier)
 KBUILD_CFLAGS += $(call cc-option, -Wno-undefined-optimized)
 KBUILD_CFLAGS += $(call cc-option, -Wno-tautological-constant-out-of-range-compare)
 KBUILD_CFLAGS += $(call cc-disable-warning, void-ptr-dereference)
+KBUILD_CFLAGS += $(call cc-disable-warning, int-conversion)
+KBUILD_CFLAGS += --prefix=$(GCC_TOOLCHAIN_DIR)$(notdir $(CROSS_COMPILE))
+KBUILD_CFLAGS += --prefix=$(GCC_TOOLCHAIN32_DIR)$(notdir $(CROSS_COMPILE_ARM32))
 
 # Quiet clang warning: comparison of unsigned expression < 0 is always false
 KBUILD_CFLAGS += $(call cc-disable-warning, tautological-compare)