`Memory` should use the target data layout information from rustc

[solson]

Memory currently has a pointer_size field. Instead, it should take tcx.data_layout to get not only pointer size, but also endianness and alignment for all primitive types. Then, Miri should diagnose unaligned memory accesses and read/write all multi-byte values with the given target endianness rather than host endianness.

• Use tcx.data_layout in Memory.
• Use target endianness.
• Detect unaligned memory accesses.