kselftest/vm: override TARGETS from arguments

kselftest/vm: override TARGETS from arguments

We can specify which testcases to build using TARGETS argument, for
example: make -C tools/testing/selftests TARGETS=vm, cause the ordinary
assignment of TARGETS in Makefile will be ignored.  So we need to use
override directive to change that.

Link: https://lkml.kernel.org/r/20220227055330.43087-1-zhouchengming@bytedance.com
Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>