arch: riscv: linker: add support for userspace

- Move kobject list after .bss
  The previous order shift kernel object address defined in prebuild
  file (which is use as reference in running time). So it was impossible
  for zephyr to check if a kernel object address was granted because
  address changed during build.

- Add support for memory shared.

- Move sdata2 section in ROM because it contains constants.

Signed-off-by: Nicolas Royer <nroyer@baylibre.com>