* Comment out unused arguments.

These last few are all that's needed to compile with -Wunused-arguments.

* Fix missing struct field initializer.

With this fix, everything compiles with -Wmissing-field-initializers.

* Add support for disabling unaligned memory accesses on x86 too.

ubsan doesn't like these because they are technically undefined
behavior, so -DGOOGLE_PROTOBUF_DONT_USE_UNALIGNED will disable them easily.

* Avoid undefined integer overflow.

ubsan catches all of these.