Makefile.debug: re-enable debug info for .S files

stable inclusion
from stable-v5.10.152
commit 3a260e9844c97c220603696b0bc225a8acf44d9c
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I73HJ0

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3a260e9844c97c220603696b0bc225a8acf44d9c



--------------------------------

This is _not_ an upstream commit and just for 5.10.y only. It is based
on commit 32ef9e50 upstream.

Alexey reported that the fraction of unknown filename instances in
kallsyms grew from ~0.3% to ~10% recently; Bill and Greg tracked it down
to assembler defined symbols, which regressed as a result of:

commit b8a90923 ("Kbuild: do not emit debug info for assembly with LLVM_IAS=1")

In that commit, I allude to restoring debug info for assembler defined
symbols in a follow up patch, but it seems I forgot to do so in

commit a66049e2 ("Kbuild: make DWARF version a choice")

Fixes: b8a90923 ("Kbuild: do not emit debug info for assembly with LLVM_IAS=1")
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Lipeng Sang <sanglipeng1@jd.com>