Makefile.debug: set -g unconditional on CONFIG_DEBUG_INFO_SPLIT

Makefile.debug: set -g unconditional on CONFIG_DEBUG_INFO_SPLIT

Dmitrii, Fangrui, and Mashahiro note:

  Before GCC 11 and Clang 12 -gsplit-dwarf implicitly uses -g2.

Fix CONFIG_DEBUG_INFO_SPLIT for gcc-11+ & clang-12+ which now need -g
specified in order for -gsplit-dwarf to work at all.

-gsplit-dwarf has been mutually exclusive with -g since support for
CONFIG_DEBUG_INFO_SPLIT was introduced in
commit 866ced950bcd ("kbuild: Support split debug info v4")
I don't think it ever needed to be.

Link: https://lore.kernel.org/lkml/20220815013317.26121-1-dmitrii.bundin.a@gmail.com/
Link: https://lore.kernel.org/lkml/CAK7LNARPAmsJD5XKAw7m_X2g7Fi-CAAsWDQiP7+ANBjkg7R7ng@mail.gmail.com/
Link: https://reviews.llvm.org/D80391
Cc: Andi Kleen <ak@linux.intel.com>
Reported-by: Dmitrii Bundin <dmitrii.bundin.a@gmail.com>
Reported-by: Fangrui Song <maskray@google.com>
Reported-by: Masahiro Yamada <masahiroy@kernel.org>
Suggested-by: Dmitrii Bundin <dmitrii.bundin.a@gmail.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>