Compile for 64 bit

These are the missing pieces referenced in #1327 for compiling for 64-bit.  They're pretty straightforward, but there are two wrinkles:

1. Defaulting to whatever the host `gcc` compiles for might make developing a 32-bit rr on a 64-bit machine challenging.  I personally do this with using a chroot, but I don't know that everybody wants to do this.
2. The `TODO` bit in `8b24c6f`.  I think there are enough things to work out before we support 32-bit programs on 64-bit hosts that defaulting to 64-bit programs only is not a big deal.