mmap map_at_zero test: handle W^X

Use kern.elfXX.allow_wx to decide whether to map W+X or W-only memory.

Future work could expand this test to add an "allow_wx" axis to the
test matrix, but I would argue that a separate test should be written,
since that's orthogonal to map_at_zero.

MFC after: 1 week
Sponsored by: Dell EMC Isilon

(cherry picked from commit 766c2466ff465b3c7c1a46be729b42a6da47de03)