Use `arch_specific_poweroff` directly instead of duplicating its
contents in Panic.cpp.
The current panic=shutdown RISC-V implementation sets the SBI
system reset reason to "System failure", so add a parameter with the
poweroff/reboot reason to arch_specific_poweroff.
This means that panic=shutdown now attempts to use the ARM PSCI,
since 6a21fcec38 forgot to add PSCI support to `__shutdown`.
This has KString, KBuffer, DoubleBuffer, KBufferBuilder, IOWindow,
UserOrKernelBuffer and ScopedCritical classes being moved to the
Kernel/Library subdirectory.
Also, move the panic and assertions handling code to that directory.
