userfaultfd: selftests: vm: pick up sanitized kernel headers

Add the usr/include subdirectory of the top-level tree to the include
path, and make sure to include headers without relative paths to make
sure the sanitized headers get picked up.  Otherwise the compiler will
not be able to find the linux/compiler.h header included by the non-
sanitized include/uapi/linux/userfaultfd.h.

While at it, make sure to only hardcode the syscall numbers on x86 and
PowerPC if they haven't been properly picked up from the headers.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Cc: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>