Makefile: Remove '--gcc-toolchain' flag

This flag was originally added to allow clang to find the GNU cross
tools in commit 785f11a ("kbuild: Add better clang cross build
support"). This flag was not enough to find the tools at times so
'--prefix' was added to the list in commit ef8c4ed ("kbuild: allow
to use GCC toolchain not in Clang search path") and improved upon in
commit ca9b31f ("Makefile: Fix GCC_TOOLCHAIN_DIR prefix for Clang
cross compilation"). Now that '--prefix' specifies a full path and
prefix, '--gcc-toolchain' serves no purpose because the kernel builds
with '-nostdinc' and '-nostdlib'.

This has been verified with self compiled LLVM 10.0.1 and LLVM 13.0.0 as
well as a distribution version of LLVM 11.1.0 without binutils in the
LLVM toolchain locations.

Link: https://reviews.llvm.org/D97902
Signed-off-by: Nathan Chancellor <[email protected]>
Reviewed-by: Fangrui Song <[email protected]>
Reviewed-by: Nick Desaulniers <[email protected]>
Tested-by: Nick Desaulniers <[email protected]>
Tested-by: Sedat Dilek <[email protected]>
Signed-off-by: Masahiro Yamada <[email protected]>

Makefile

@@ -581,10 +581,6 @@ ifneq ($(CROSS_COMPILE),)
 CLANG_FLAGS	+= --target=$(notdir $(CROSS_COMPILE:%-=%))
 GCC_TOOLCHAIN_DIR := $(dir $(shell which $(CROSS_COMPILE)elfedit))
 CLANG_FLAGS	+= --prefix=$(GCC_TOOLCHAIN_DIR)$(notdir $(CROSS_COMPILE))
-GCC_TOOLCHAIN	:= $(realpath $(GCC_TOOLCHAIN_DIR)/..)
-endif
-ifneq ($(GCC_TOOLCHAIN),)
-CLANG_FLAGS	+= --gcc-toolchain=$(GCC_TOOLCHAIN)
 endif
 ifneq ($(LLVM_IAS),1)
 CLANG_FLAGS	+= -no-integrated-as