tests/posixshm: Check for hardware support in largepage_pkru
Approved by: re (cperciva)
MFC after: 3 days
Fixes: ca87c0b8e396 ("pkru: Fix handling of 1GB largepage mappings")
(cherry picked from commit 75c66218401c00f4728d5312e2b933b8d3aefde1)
(cherry picked from commit 8988b4de4e3b85baa97fd76254e00da7d3304b35)