x86: make _PAGE_HIDDEN conditional

Only _PAGE_HIDDEN when CONFIG_KMEMCHECK is defined, otherwise set it
to 0.  Allows later cleanups.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>

[rebased for mainline inclusion]
Signed-off-by: Vegard Nossum <vegard.nossum@gmail.com>