Mostly-boilerplate code to start x86-64 support

Remove uses of bfd_architecture in the LibASMIR interface, since
it's a compatibility issue and doesn't distinguish 32-bit from
64-bit x86. Some simple uses of insn_test and test_insn now work,
but a number of important things are still missing or broken:
- The 64-bit code doesn't support flags yet. This needs code very
similar to 32-bit x86, so it should ideally be refactored to be
mostly shared.
- Several parts of Vine and FuzzBALL assume addresses are 32 bits.
- The Linux loader doesn't know about 64-bit ELF.
- Linux system calls are not supported in 64-bit.