selftests: lib.mk Fix individual test builds

In commit a8ba798b ("selftests: enable O and KBUILD_OUTPUT"), added
support to generate compile targets in a user specified directory. OUTPUT
variable controls the location which is undefined when tests are built in
the test directory or with "make -C tools/testing/selftests/x86".

make -C tools/testing/selftests/x86/
make: Entering directory '/lkml/linux_4.11/tools/testing/selftests/x86'
Makefile:44: warning: overriding recipe for target 'clean'
../lib.mk:51: warning: ignoring old recipe for target 'clean'
gcc -m64 -o /single_step_syscall_64 -O2 -g -std=gnu99 -pthread -Wall  single_step_syscall.c -lrt -ldl
/usr/bin/ld: cannot open output file /single_step_syscall_64: Permission denied
collect2: error: ld returned 1 exit status
Makefile:50: recipe for target '/single_step_syscall_64' failed
make: *** [/single_step_syscall_64] Error 1
make: Leaving directory '/lkml/linux_4.11/tools/testing/selftests/x86'

Same failure with "cd tools/testing/selftests/x86/;make" run.

Fix this with a change to lib.mk to define OUTPUT to be the pwd when
MAKELEVEL is 0. This covers both cases mentioned above.

Reported-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>