Makefile: Remove '--gcc-toolchain' flag

This flag was originally added to allow clang to find the GNU cross
tools in commit 785f11aa ("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 ef8c4ed9 ("kbuild: allow
to use GCC toolchain not in Clang search path") and improved upon in
commit ca9b31f6 ("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 <nathan@kernel.org>
Reviewed-by: Fangrui Song <maskray@google.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>