tools selftests: Fix 'clean' target with make 3.81

tools selftests: Fix 'clean' target with make 3.81

commit 60df4642a83546fa6ea8286f5094ce8c0906c3ec upstream.

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>
Cc: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>