Strict thread stack isolation and sharing

[Utkarsh Rai]

Following is the complete set of changes for thread stack isolation and
sharing.

Configuration for thread stack isolation.
> The configuration for thread stack isolation is based upon the new build
system.
> The RTEMS_THREAD_STACK_PROTECTION option needs to be specified and set to
'True'
   for enabling thread stack protection.
> I have used the realview_pbx_a9 BSP which already has support for small
pages. We need different page configuration options based upon whether the
architecture supports MMU or MPU.

Application requirements for stack sharing -
 > Naming for shared memory objects is done in the application and the name
follows a fixed naming pattern ( "/taskfs/" ), this is used to
differentiate between a normal mmap operation and a stack sharing operation.
> We need to explicitly allocate stack memory from the application for
stack sharing, and then set through pthread_attr_setstack*()
> Any application has to specify a series of repetitive steps (shm_open,
ftruncate, mmap) for sharing a particular thread-stack. Maybe this can be
wrapped under a function ((rtems_share_stack() ?) ) and we only make a call
to that function every time we have to share a thread stack.
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