selftests/vm: minor cleanup: Makefile and gup_test.c

A few cleanups that don't deserve separate patches, but that also should
not clutter up other functional changes:

1. Remove an unnecessary #include <prctl.h>

2. Restore the sorted order of TEST_GEN_FILES.

3. Add -lpthread to the common LDLIBS, as it is harmless and several
   tests use it. This gets rid of one special rule already.

Link: https://lkml.kernel.org/r/20201026064021.3545418-5-jhubbard@nvidia.com


Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Cc: Jérôme Glisse <jglisse@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Ralph Campbell <rcampbell@nvidia.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>