When the OS_TaskCreate() function is called with a non-NULL stack pointer,
the POSIX implementation now should use this pointer as intended.  Previously
it was always allocating a new stack buffer, ignoring the passed-in pointer.

NOTE: When using this feature, the caller must use caution to make sure the
buffer being passed in is large enough to use as a task stack.  Particularly
when running under a desktop Linux/Glibc environment, the infrastructure
puts a considerably large object on the stack.  For example, in the
osal-core-tests, the stack size was too small.