Move prctl tests from Documentation/prctl to selftests/prctl.

Remove prctl from Makefile to move the test. Update prctl Makefile to work
under selftests. prctl will not be run as part of selftests suite and will
not be included in install targets. They can be built separately for now.
Acked-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>