Use MAP_STACK in qemu_alloc_stack() on OpenBSD.

Added to our 6.4 release.

MAP_STACK      Indicate that the mapping is used as a stack.  This
               flag must be used in combination with MAP_ANON and
               MAP_PRIVATE.

Implement MAP_STACK option for mmap().  Synchronous faults (pagefault and
syscall) confirm the stack register points at MAP_STACK memory, otherwise
SIGSEGV is delivered. sigaltstack() and pthread_attr_setstack() are modified
to create a MAP_STACK sub-region which satisfies alignment requirements.
Observe that MAP_STACK can only be set/cleared by mmap(), which zeroes the
contents of the region -- there is no mprotect() equivalent operation, so
there is no MAP_STACK-adding gadget.

Signed-off-by: Brad Smith <brad@comstyle.com>
Reviewed-by: Kamil Rytarowski <n54@gmx.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20181019125239.GA13884@humpty.home.comstyle.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>