kbuild: Add ability to test Clang's integrated assembler

There are some people interested in experimenting with Clang's
integrated assembler. To make it easy to do so without source
modification, allow the user to specify 'AS=clang' as part of the
make command to avoid adding '-no-integrated-as' to the {A,C}FLAGS.

Link: https://github.com/ClangBuiltLinux/linux/issues/577Suggested-by: Dmitry Golovin <dima@golovin.in>
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

Makefile

@@ -527,7 +527,9 @@ endif
 ifneq ($(GCC_TOOLCHAIN),)
 CLANG_FLAGS	+= --gcc-toolchain=$(GCC_TOOLCHAIN)
 endif
+ifeq ($(shell $(AS) --version 2>&1 | head -n 1 | grep clang),)
 CLANG_FLAGS	+= -no-integrated-as
+endif
 CLANG_FLAGS	+= -Werror=unknown-warning-option
 KBUILD_CFLAGS	+= $(CLANG_FLAGS)
 KBUILD_AFLAGS	+= $(CLANG_FLAGS)