Makefile: Fix GCC_TOOLCHAIN_DIR prefix for Clang cross compilation

BugLink: https://bugs.launchpad.net/bugs/1889928

commit ca9b31f6 upstream.

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
https://github.com/llvm/llvm-project/commit/3452a0d8c17f7166f479706b293caf6ac76ffd90)
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>
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: https://github.com/ClangBuiltLinux/linux/issues/1099Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
[nc: Adjust context, CLANG_FLAGS does not exist in 4.4]
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

Makefile

@@ -617,7 +617,7 @@ ifeq ($(cc-name),clang)
 ifneq ($(CROSS_COMPILE),)
 CLANG_TARGET	:= --target=$(notdir $(CROSS_COMPILE:%-=%))
 GCC_TOOLCHAIN_DIR := $(dir $(shell which $(CROSS_COMPILE)elfedit))
-CLANG_PREFIX	:= --prefix=$(GCC_TOOLCHAIN_DIR)
+CLANG_PREFIX	:= --prefix=$(GCC_TOOLCHAIN_DIR)$(notdir $(CROSS_COMPILE))
 GCC_TOOLCHAIN	:= $(realpath $(GCC_TOOLCHAIN_DIR)/..)
 endif
 ifneq ($(GCC_TOOLCHAIN),)