Kbuild: do not emit debug info for assembly with LLVM_IAS=1

commit b8a9092330da2030496ff357272f342eb970d51b upstream.

Clang's integrated assembler produces the warning for assembly files:

warning: DWARF2 only supports one section per compilation unit

If -Wa,-gdwarf-* is unspecified, then debug info is not emitted for
assembly sources (it is still emitted for C sources).  This will be
re-enabled for newer DWARF versions in a follow up patch.

Enables defconfig+CONFIG_DEBUG_INFO to build cleanly with
LLVM=1 LLVM_IAS=1 for x86_64 and arm64.

Cc: <stable@vger.kernel.org>
Link: ClangBuiltLinux/linux#716
Reported-by: Dmitry Golovin <dima@golovin.in>
Reported-by: Nathan Chancellor <natechancellor@gmail.com>
Suggested-by: Dmitry Golovin <dima@golovin.in>
Suggested-by: Nathan Chancellor <natechancellor@gmail.com>
Suggested-by: Sedat Dilek <sedat.dilek@gmail.com>
Reviewed-by: Fangrui Song <maskray@google.com>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
[nd: backport to avoid conflicts from:
  commit 10e68b02c861 ("Makefile: support compressed debug info")
  commit 7b16994437c7 ("Makefile: Improve compressed debug info support detection")
  commit 695afd3d7d58 ("kbuild: Simplify DEBUG_INFO Kconfig handling")]
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Change-Id: Ia4da22fe1254ea433d4d346fc24158d66663c11a

Makefile

@@ -846,8 +846,11 @@ KBUILD_CFLAGS   += $(call cc-option, -gsplit-dwarf, -g)
 else
 KBUILD_CFLAGS	+= -g
 endif
+ifneq ($(LLVM_IAS),1)
 KBUILD_AFLAGS	+= -Wa,-gdwarf-2
 endif
+endif
+
 ifdef CONFIG_DEBUG_INFO_DWARF4
 KBUILD_CFLAGS	+= $(call cc-option, -gdwarf-4,)
 endif