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

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

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 b8a9092330da ("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 a66049e2cf0e ("Kbuild: make DWARF version a choice")

Link: https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=31bf18645d98b4d3d7357353be840e320649a67d
Fixes: b8a9092330da ("Kbuild: do not emit debug info for assembly with LLVM_IAS=1")
Reported-by: Alexey Alexandrov <[email protected]>
Reported-by: Bill Wendling <[email protected]>
Reported-by: Greg Thelen <[email protected]>
Reviewed-by: Nathan Chancellor <[email protected]>
Suggested-by: Masahiro Yamada <[email protected]>
Signed-off-by: Nick Desaulniers <[email protected]>
Signed-off-by: Masahiro Yamada <[email protected]>