This adds the standard window-flush ISR to Edisoft RTEMS 4.8 builds,
allowing the custom SPARC setjmp/longjmp implementation to use "ta 3" to
instigate a window flush.  For non-Edisoft builds "ta 3" can be used
without any special additions because it already exists.  For Edisoft
builds the function sparc_install_ta_3_window_flush_isr() must be called
before any MicroPython functions are called, in order to install the custom
handler.

With this patch the implementation of sparc_window_flush_trap_handler
matches the standard implementation.