kbuild: move cmd_cc_o_c and cmd_as_o_S to scripts/Malefile.lib

The cmd_cc_o_c and cmd_as_o_S macros are duplicated in
scripts/Makefile.{build,modfinal,vmlinux}.

This commit factors them out to scripts/Makefile.lib.

No functional changes are intended.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>