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

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]>