selftests: Set CC using CROSS_COMPILE once in lib.mk

This avoids repeating the logic in every Makefile. We mimic the
top-level Makefile and use $(CROSS_COMPILE)gcc.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>