tools selftests: Fix 'clean' target with make 3.81

Make 3.81 doesn't have the 'undefine' command. Using undefine
to clear LDFLAGS fails when make version 3.81 is used. Fix it
to use override to clear LDFLAGS.

Tested-by: Shuah Khan <shuahkh@osg.samsung.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Link: http://lkml.kernel.org/r/20150514151225.GH23588@kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>