Alignment changes

[jgriffiths]

• Fix alignment checks for hashes which would slightly pessimize strong alignment targets.
• Align pk_h hash output for better hash performance.
• Mark arm 32bit as not supporting unaligned access.

The last commit fixes a crash on Rasbian reported by @jmastr, thanks for the bug report!

Fixes https://github.com/ElementsProject/libwally-core/issues/396