x86: move x86_userspace_thread_exit() from user stack to commpage

x86_userspace_thread_exit() is a stub originally placed at the bottom of
each thread user stack that ensures any thread invokes exit_thread() upon
returning from its main higher level function.

Putting anything that is expected to be executed on a stack causes problems
when implementing data execution prevention. Code of x86_userspace_thread_exit()
is now moved to commpage which seems to be much more appropriate place for it.
x86: enable data execution prevention

Set execute disable bit for any page that belongs to area with neither
B_EXECUTE_AREA nor B_KERNEL_EXECUTE_AREA set.

In order to take advanage of NX bit in 32 bit protected mode PAE must be
enabled. Thus, from now on it is also enabled when the CPU supports NX bit.

vm_page_fault() takes additional argument which indicates whether page fault
was caused by an illegal instruction fetch.