use volatile access to L4 TCRs in userspace

GCC 4.8 was found to elide a L4_Set_Pager() when the TCR was reset after a
memset(), despite the intent being to change the TCR during the memset() and
restore it only afterward. With this it no longer does, restoring correct
behaviour.
test: thread: add a millisecond sleep to halt_on_missing_pager

This ensures that the test subject enters the faulting segment before
analysis.