tests: fatal/exception: pin stack before stack overflow test

For hardware stack overflow test, pin the whole stack if
demand paging is enabled and generic sections are not all present
at boot. The whole stack may not be in memory at the time of
test, which would result in double fault (exception being
handled + page fault). So make sure the stack is in physical
memory and mapped before doing any tests.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>