tests/tcg/multiarch: expand system memory test to cover more

Expand the memory test to cover move of the softmmu code. Specifically
we:

  - improve commentary
  - add some helpers (for later BE support)
  - reduce boiler plate into helpers
  - add signed reads at various sizes/offsets
  - required -DCHECK_UNALIGNED

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>