Use the POSIX-specified stack_t type as the argument to sigaltstack()
rather than the legacy struct sigaltstack. This allows us to compile
on MacOSX with --with-coroutine=sigaltstack.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>