linker: Add devnull memory for cortex_m and riscv32

Added memory region which is intended to be removed from the final
binary but may be using in byproduct to extract data used by the
host tools (e.g. strings used by logging).

Add devnull region to Cortex-M and RISCV linker scripts.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>