[lld] r220730 - TMP: fix readN & writeN to not encourage UB

[Tim Northover]

> TMP: fix readN & writeN to not encourage UB

Bother, forgot to edit the commit message. This isn't actually TMP.
The original problem was that pointers were being cast to uint32_t*
and dereferenced without any regard for alignment (let alone TBAA).
This should fix the obvious cases, but I still wouldn't necessarily
guarantee we've got it right.

See D5811 for some more details.

Tim.