selftests: lib.mk: kselftest and kselftest-clean fail for make O=dir case

kselftest and kselftest-clean targets fail when object directory is
specified to relocate objects. Main Makefile make O= path clears the
built-in defines LINK.c, COMPILE.S, LINK.S, and RM that are used in
lib.mk to build and clean targets. Define them.

Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>