arch/riscv: support CONFIG_CODE_DATA_RELOCATION

This implements support for relocating code to chosen memory regions via
the `zephyr_code_relocate` CMake function for RISC-V SoCs. ARM-specific
assumptions that were made by gen_relocate_app.py need to be corrected,
in particular not assuming any particular name for the default RAM
section (which is 'SRAM' for most ARM pltaforms) and not assuming 32-bit
pointers (so the test works on RV64).

Signed-off-by: Peter Marheine <pmarheine@chromium.org>