MemArena: Fix the launching of non-Wii games

When we cleaned up the code to calculate the shm_position and total_mem
in one step, we sometimes skipped over certain views because they were
Wii-only. When looking at the total memory, we'd look at the last field,
whether or not it was skipped. Since Wii-only fields are the last view,
this meant that the shm_position was 0, since it was skipped, causing us
to map a 0-sized field. Fix this by explicitly returning the total size
from MemoryMap_InitializeViews.

Additionally, the shm_position was being calculated incorrectly because
it was adding up the shm_position *before* the mirror, rather than after
it. Fix this by adopting a scheme similar to what we had before.