64-bit preload support

This series hacks `librrpreload` into shape to at least compile (but not link, see #1324) for 64-bit.  It also makes rr itself link on 64-bit, though probably not work (`Registers` still needs some 64-bit love, there are undoubtedly a few more 32-bit-isms floating around, and 64-bit rr won't work without the monkeypatching provided by `librrpreload` anyway, so...).