build: Check that mlockall() exists

Instead of assuming that all POSIX platforms provide mlockall(),
test for it in configure. If the host doesn't provide this platform
then os_mlock() will fail -ENOSYS, as it does already on Windows.

This is necessary for Haiku, which does not have mlockall().

Signed-off-by: David Carlier <devnexen@gmail.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20200703145614.16684-4-peter.maydell@linaro.org
[PMM: Expanded commit message; rename to HAVE_MLOCKALL]
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>