RTEMS | cpukit/score/cpu/microblaze: Use a reserved opcode (!1577)

[Aaditya Agarwal (@aaditya8979)]

Aaditya Agarwal created a merge request: https://gitlab.rtems.org/rtems/rtos/rtems/-/merge_requests/1577

Project:Branches: aaditya8979/rtems:fix-5759 to rtems/rtos/rtems:main
Author: Aaditya Agarwal

_CPU_Instruction_illegal() emitted `.word 0x0`. Major opcode 0 is `add`, so the word is `add r0, r0, r0` and retires without an exception unless the core is synthesized with `C_OPCODE_0x0_ILLEGAL`. Both this option and `C_ILL_OPCODE_EXCEPTION` are synthesis options of the soft core, and QEMU defaults both to false.

Use major opcode 0x3f (`0xfc000000`), which is unallocated in the MicroBlaze architecture. An invalid major opcode requires only `C_ILL_OPCODE_EXCEPTION` to raise an illegal opcode exception.

Closes #5759

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View it on GitLab: https://gitlab.rtems.org/rtems/rtos/rtems/-/merge_requests/1577
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