soc: riscv: privilege: define __soc_handle_irq as weak symbol

Define __soc_handle_irq as a weak symbol in the common RISC-V privileged
instruction set SoC support.

This allows overriding __soc_handle_irq for SoCs which are not fully
compliant with the RISC-V privileged specification.

Signed-off-by: Henrik Brix Andersen <henrik@brixandersen.dk>