arch/riscv: Advance the PC after handling misaligned load/store

arch/riscv: Advance the PC after handling misaligned load/store

After emulating an instruction in the misaligned load/store handler, we
need to increment the program counter by the size of instruction.
Otherwise the same instruction is executed (and emulated) again and again.

While were at it: Also return early in the unlikely case that the
faulting instruction is not 16 or 32 bits long, and be more explicit
about the return values of fetch_*bit_instruction.

Tested by Philipp Hug, using the linuxcheck payload.

Fixes: cda59b56ba ("riscv: update misaligned memory access exception handling")
Change-Id: Ie2dc0083835809971143cd6ab89fe4f7acd2a845
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-on: https://review.coreboot.org/28617
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Philipp Hug <philipp@hug.cx>
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>