fix: implement real irq_save/irq_restore for ARM, RISC-V, MIPS

fix: implement real irq_save/irq_restore for ARM, RISC-V, MIPS

spinlock.h had no-op irq_save/irq_restore for non-x86 architectures,
meaning spin_lock_irqsave would not actually disable interrupts on
ARM, RISC-V, or MIPS — breaking all critical sections.

Add proper implementations:
- ARM: mrs/msr CPSR with cpsid i
- RISC-V: csrrci/csrsi mstatus MIE bit
- MIPS: mfc0/di/ei on CP0 Status IE bit

Passes: make, cppcheck, QEMU smoke test.