Building cargo on ARM

[japaric]

• Some C dependencies pass -m64 (which is not available in ARM) to gcc
  • [x] flate2-rs alexcrichton/flate2-rs#1
  • [x] curl-sys alexcrichton/curl-rust#2
  • [x] libssh2-static-sys alexcrichton/libssh2-static-sys#2
  • [x] libgit2 alexcrichton/git2-rs#13
• [x] Build cargo statically #6
• Patch dl-snapshot.py to fetch an unofficial nightly/snapshot
• [x] Create script to build cargo from an unofficial cargo nightly (using --local-cargo) 6274ba2
• [x] Update README