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

This is _not_ an upstream commit and just for 5.15.y only. It is based
on commit 32ef9e5054 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 b8a9092330 ("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 a66049e2cf ("Kbuild: make DWARF version a choice")

Fixes: b8a9092330 ("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>

Makefile

@@ -870,7 +870,9 @@ else
 DEBUG_CFLAGS	+= -g
 endif
 
-ifndef CONFIG_AS_IS_LLVM
+ifdef CONFIG_AS_IS_LLVM
+KBUILD_AFLAGS	+= -g
+else
 KBUILD_AFLAGS	+= -Wa,-gdwarf-2
 endif