selftests: add default rules for c source file

There are difference rules for compiling c source file in different
testcases. In order to enable KBUILD_OUTPUT support in later patch,
this patch introduce the default rules in
"tools/testing/selftest/lib.mk" and remove the existing rules in each
testcase.

Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Bamvor Jian Zhang <bamvor.zhangjian@linaro.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>