build: add basic arm64 support

This commit adds basic arm64 support to the build. Building the bundled openssl is disabled pending an upgrade to openssl 1.2. The currently bundled version has some hand-rolled assembly that is 32 bits only.

The first commit is #1027.

It's not all roses but at least it compiles now.

R=@rvagg, /cc @shigeki @silverwind