Fix SEC_STAT.IRM bit handling in ARC SEM interrupt entry/exit

[vishwamartur]

Related to #331

Fix the Memory Read Protection Violation from Secure MPU exception on exit from interrupt.

• Add code to store SEC_STAT.IRM bit in INTERRUPT_PROLOGUE macro.
• Add code to restore SEC_STAT.IRM bit in INTERRUPT_EPILOGUE macro.
• Modify portable/ThirdParty/GCC/ARC_EM_HS/arc_support.s to include the changes in the interrupt entry/exit macros.

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