selftests: vm: Fix 64-bit test builds for powerpc64le

Some tests are built only for 64-bit systems. This makes
sure that these tests are built for both big and little
endian variants of powerpc64.

Fixes: 7549b336 ("selftests: vm: Build/Run 64bit tests only on 64bit arch")
Reviewed-by: Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>
Signed-off-by: Sandipan Das <sandipan@linux.ibm.com>
Tested-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>