#define 'HAVE_UNALIGNED_ACCESS' and [-Wcast-align] warnings on 32bit armv7l

[Chirimen-Jako]

• May I have a question when I build Lightning on 32bit armv7l?
  The issue is mainly happen in ccan. but the other part cause same warnings.
  Detailed Info

Issue and Steps to Reproduce

$ make clean
$ ./configure
$ make

getinfo output

$ lightning-cli getinfo
{
   "id" : "036c518bbbf7afd2dd00ec145b501b782f1ec4112928e650db78319c52ee05b7fb",
   "alias" : "ChirimenJako",
   "color" : "00fed9",
   "num_peers" : 2,
   "num_pending_channels" : 0,
   "num_active_channels" : 2,
   "num_inactive_channels" : 0,
   "address" : [],
   "binding" : [
      {
         "type" : "ipv4",
         "address" : "0.0.0.0",
         "port" : 9735
      }
   ],
   "version" : "v0.7.1-230-gfc024f8",
   "blockheight" : 588981,
   "network" : "bitcoin",
   "msatoshi_fees_collected" : 3001,
   "fees_collected_msat" : "3001msat"
}

[ZmnSCPxj]

It is known currently that C-Lightning has trouble with ARM variants that SIGBUS (bus error) on unaligned access. #2818

Some ARM variants, like iMX6, are known to work with unaligned access: #2818 (comment)

Unfortunately, the unaligned access is in our dependent library CCAN, so we need to pressure @rustyrussell to look into this further.