fix: replace x86 'cli; hlt' asm in syscall.c with HAL calls

fix: replace x86 'cli; hlt' asm in syscall.c with HAL calls

syscall.c used raw x86 inline assembly ('cli; hlt') in the exit
syscall idle loop. This would not compile on ARM/RISC-V/MIPS.

Add hal_cpu_disable_interrupts() to the HAL CPU interface with
implementations for x86 (cli) and stubs for other architectures.
Replace the raw asm with hal_cpu_disable_interrupts() + hal_cpu_idle().

Passes: make, cppcheck, QEMU smoke test.