Clean up unaligned access routines in compiler.h

[Matt Turner]

These patches build upon my previous patchset to clean mem barrier
macros.

The first factors out the unaligned access code into a common section
so that it's easy to see what can be combined. The second combines
common code paths.

I've changed the argument and return types to uint{16,32,64}_t, to
make them semantically correct, but it's possible that I've broken
something in the process.