kbuild: move -gsplit-dwarf, -gdwarf-4 option tests to Kconfig

CONFIG_DEBUG_INFO_SPLIT and CONFIG_DEBUG_INFO_DWARF4 enable extra
dwarf options if supported. You never know if they are really enabled
since Makefile may silently turn them off.

The actual behavior will match to the kernel configuration by
testing those compiler flags in the Kconfig stage.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>