riscv32: added a generic linker script for the riscv32 platform

Added a linker script that shall be common to most riscv SOCs.
Linker script also accounts for execution in place in ROM, when
CONFIG_XIP is set.

Nonetheless, riscv32 SOCs (like pulpino) requiring a different
system layout can still define their own linker script.

Change-Id: I3ad670446d439772c29a8204e307ac79643dc650
Signed-off-by: Jean-Paul Etienne <fractalclone@gmail.com>