libnvdimm: Enable unit test infrastructure compile checks

The infrastructure to mock core libnvdimm routines for unit testing
purposes is prone to bitrot relative to refactoring of that core.  Arrange
for the unit test core to be built when CONFIG_COMPILE_TEST=y.  This does
not result in a functional unit test environment, it is only a helper for
0day to catch unit test build regressions.

Note that there are a few x86isms in the implementation, so this does not
bother compile testing this architectures other than 64-bit x86.

Link: https://lore.kernel.org/r/156763690875.2556198.15786177395425033830.stgit@dwillia2-desk3.amr.corp.intel.com
Reported-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>