leon: Make sparc_install_ta_3_window_flush_isr always available.
The header sparcisr.h is part of the ABI and may be included by code which does not define RTEMS_4_8_EDISOFT yet uses 4.8 Edisoft. In such cases the definition of sparc_install_ta_3_window_flush_isr will be inconsistent. So just make this function always available, and provide the correct definition in the .c file (which must have RTEMS_4_8_EDISOFT defined if that toolchain is used).
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