64-bit length fields on ARM don't work

ARM only allows aligned accesses.  GCC generated unaligned accesses for the copy buffer, but ARM forces alignment, causing the first two bytes to be trampled.  I changed the mechanism to create two temporaries and memcpy them in.  It now works on ARM (and x86) for large websocket chunks.
Merge pull request #449 from ursine/patch-1

64-bit length fields on ARM don't work