bugfix: correct computation of stack size on Mac Posix port (#816)

bugfix: correct computation of stack size on Mac Posix port (#816)

Aligns the stack end to a page boundary before computing its
size, since the size depends on both the start and end.

The original change which introduced stack alignment (#674)
only worked for cases where the round + trunc operation would
wind up within the same area, but would lead to segfaults in
other cases.

Also adds a typecast to the `mach_vm_round_page()` call, as
it is actually a macro which casts to `mach_vm_offset_t` and
the result here is used as a `StackType_t` pointer.

Tested on ARM64 and Intel MacOS, as well as ARM64 and Intel
Linux.  The test code included a single-task case, as well
as a case with two tasks passing queue messages.