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

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: <[email protected]>
Link: https://github.com/ClangBuiltLinux/linux/issues/716
Reported-by: Dmitry Golovin <[email protected]>
Reported-by: Nathan Chancellor <[email protected]>
Suggested-by: Dmitry Golovin <[email protected]>
Suggested-by: Nathan Chancellor <[email protected]>
Suggested-by: Sedat Dilek <[email protected]>
Reviewed-by: Fangrui Song <[email protected]>
Reviewed-by: Nathan Chancellor <[email protected]>
Signed-off-by: Nick Desaulniers <[email protected]>
Signed-off-by: Masahiro Yamada <[email protected]>

Makefile

@@ -828,7 +828,9 @@ else
 DEBUG_CFLAGS	+= -g
 endif
 
+ifneq ($(LLVM_IAS),1)
 KBUILD_AFLAGS	+= -Wa,-gdwarf-2
+endif
 
 ifdef CONFIG_DEBUG_INFO_DWARF4
 DEBUG_CFLAGS	+= -gdwarf-4