osdep: Move memalign-related functions to their own header

Move the various memalign-related functions out of osdep.h and into
their own header, which we include only where they are used.
While we're doing this, add some brief documentation comments.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20220226180723.1706285-10-peter.maydell@linaro.org
diff --git a/qom/object.c b/qom/object.c
index 9f7a331..4048a2f 100644
--- a/qom/object.c
+++ b/qom/object.c
@@ -16,6 +16,7 @@
 #include "qom/object.h"
 #include "qom/object_interfaces.h"
 #include "qemu/cutils.h"
+#include "qemu/memalign.h"
 #include "qapi/visitor.h"
 #include "qapi/string-input-visitor.h"
 #include "qapi/string-output-visitor.h"