Moved the ARC ISR enter callout onto the system stack (#700)

_tx_thread_context_save() calls _tx_execution_isr_enter when
TX_ENABLE_EXECUTION_CHANGE_NOTIFY is defined. On the path where an interrupt
preempted a running thread, that call was made before the switch to the system
stack, so the 32-byte call frame and the whole stack footprint of the callout
were taken from the interrupted thread's stack, on top of the 160-byte interrupt
frame the port had already allocated there.

The callout is supplied by the application, so its stack usage is not bounded by
ThreadX, and it is charged to every thread that happens to be running when an
interrupt arrives.

The two other callout sites in the same routine, the nested save and the idle
system save, already run on the system stack, as does the _tx_execution_isr_exit
call in _tx_thread_context_restore. _tx_thread_schedule was reordered in 6.1.9
so that _tx_execution_thread_enter runs on the system stack rather than the
thread stack; the same reorder was never applied to the context save.

The switch to the system stack now happens before the callout in the ARCv2_EM,
ARC_HS and SMP ARC_HS ports. _tx_thread_context_fast_save is unchanged because
the fast interrupt path never switches stacks by design.

Fixes #149

Assisted-by: Copilot (Opus 5) <[email protected]>

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