accel/tcg: Reorg system mode load helpers

Instead of trying to unify all operations on uint64_t, pull out
mmu_lookup() to perform the basic tlb hit and resolution.
Create individual functions to handle access by size.

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