bsp/leon3: Unmask IPI only on secondary processor
score: SMP initialization and shutdown changes

Rename _SMP_Request_other_cores_to_perform_first_context_switch() into
_SMP_Request_start_multitasking() since this requests now a multitasking
start on all configured and available processors.  The name corresponds
_Thread_Start_multitasking() and
_SMP_Start_multitasking_on_secondary_processor() actions issued in
response to this request.  Move in source file to right place.

Rename PER_CPU_STATE_READY_TO_BEGIN_MULTITASKING into
PER_CPU_STATE_READY_TO_START_MULTITASKING.

Rename PER_CPU_STATE_BEGIN_MULTITASKING into
PER_CPU_STATE_REQUEST_START_MULTITASKING.

Rename _SMP_Request_other_cores_to_shutdown() into
_SMP_Request_shutdown().

Add a per-CPU state lock to protect all changes.  This was necessary to
offer a controlled shutdown of the system (atomic read/writes alone are
not sufficient for this kind of synchronization).

Add documentation for Per_CPU_State.

Delete debug output.

New tests smptests/smpfatal01 and smptests/smpfatal02.
score: Add and use _Giant_Drop()

New test smptests/smpfatal03.
libnetworking: Typo
score: Add _Atomic_Fence()
bsps: SMP support for generic interrupt support
rtems: Add RTEMS_INTERRUPT_REPLACE

A new option RTEMS_INTERRUPT_REPLACE is introduced that permits updating
the first interrupt handler for the registered interrupt vector and
matching argument.  If no match is found, the install function fails
with RTEMS_UNSATISFIED.

The Interrupt Manager Extension offers interrupt handlers with an
argument pointer.  It is impossible to update two words (handler and
argument) atomically on most architectures.  In order to avoid an SMP
lock in bsp_interrupt_handler_dispatch() which would degrade the
interrupt response time an alternative must be provided that makes it
possible to tear-down interrupt sources without an SMP lock.

Add RTEMS_INTERRUPT_REPLACE option to Interrupt Manager Extension.  This
enables a clean tear-down of interrupt sources on SMP configurations.
Instead of an interrupt handler removal a replacement handler can be
installed to silence an interrupt source.  This can be used in contexts
that allow no sophisticated synchronization (e.g. in atexit() or fatal
handlers).
tmtests/tm26: Fix deadlock for SMP configurations

Some _Context_Switch() invocations end up in _Thread_Handler().  Create
the right context for this function.
score: Delete _Thread_Dispatch_set_disable_level()

This function was only used in some tests and can be replaced with other
functions.