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: 785f11a ("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

@@ -783,7 +783,7 @@ ifeq ($(cc-name),clang)
 ifneq ($(CROSS_COMPILE),)
 CLANG_TRIPLE    ?= $(CROSS_COMPILE)
 CLANG_TARGET	:= --target=$(notdir $(CLANG_TRIPLE:%-=%))
-GCC_TOOLCHAIN_DIR := $(dir $(shell which $(LD)))
+GCC_TOOLCHAIN_DIR := $(dir $(shell which $(CROSS_COMPILE)elfedit))
 CLANG_PREFIX	:= --prefix=$(GCC_TOOLCHAIN_DIR)
 GCC_TOOLCHAIN	:= $(realpath $(GCC_TOOLCHAIN_DIR)/..)
 endif