[feature] `poweroff` and `reboot` functionality

[ddrcode]

Explore and implement poweroff/reboot functions, starting from Qemu.
Poweroff is especially important, as it would allow to implement some test runner and possibility to run multiple tests one after another.

[ddrcode]

One of the options could be semihosting. See:

• RISC-V Semihosting
• Emulation

Code for ARM (couldn't find any for RISC-V yet)

register int reg0 asm("r0");
register int reg1 asm("r1");

reg0 = 0x18;    // angel_SWIreason_ReportException
reg1 = 0x20026; // ADP_Stopped_ApplicationExit

asm("svc 0x00123456");  // make semihosting call