kbuild: clang: choose GCC_TOOLCHAIN_DIR not on LD

commit ad15006cc78459d059af56729c4d9bed7c7fd860 upstream.

This causes an issue when trying to build with `make LD=ld.lld` if
ld.lld and the rest of your cross tools aren't in the same directory
(ex. /usr/local/bin) (as is the case for Android's build system), as the
GCC_TOOLCHAIN_DIR then gets set based on `which $(LD)` which will point
where LLVM tools are, not GCC/binutils tools are located.

Instead, select the GCC_TOOLCHAIN_DIR based on another tool provided by
binutils for which LLVM does not provide a substitute for, such as
elfedit.

Fixes: 785f11aa595b ("kbuild: Add better clang cross build support")
Link: ClangBuiltLinux/linux#341
Suggested-by: Nathan Chancellor <[email protected]>
Reviewed-by: Nathan Chancellor <[email protected]>
Tested-by: Nathan Chancellor <[email protected]>
Signed-off-by: Nick Desaulniers <[email protected]>
Signed-off-by: Masahiro Yamada <[email protected]>
Signed-off-by: Nathan Chancellor <[email protected]>
Signed-off-by: Sasha Levin <[email protected]>

Makefile

@@ -695,7 +695,7 @@ KBUILD_CFLAGS += $(stackp-flag)
 ifeq ($(cc-name),clang)
 ifneq ($(CROSS_COMPILE),)
 CLANG_TARGET	:= -target $(notdir $(CROSS_COMPILE:%-=%))
-GCC_TOOLCHAIN	:= $(realpath $(dir $(shell which $(LD)))/..)
+GCC_TOOLCHAIN	:= $(realpath $(dir $(shell which $(CROSS_COMPILE)elfedit))/..)
 endif
 ifneq ($(GCC_TOOLCHAIN),)
 CLANG_GCC_TC	:= -gcc-toolchain $(GCC_TOOLCHAIN)