Basic x86_64 support

This set of commits implements basic x86_64 guest support for S2E. Some plugins still need to be ported. Used the ./grade-this-exam demo for testing.

Tested:
- Demo with 32 bit guest running under i386-s2e-softmmu qemu
- Demo with 32 bit guest running under x86_64-s2e-softmmu qemu
- Demo with 64 bit guest running x86_64-s2e-softmmu qemu with 64 bit test binaries
- Demo with 64 bit guest running x86_64-s2e-softmmu qemu with 32 bit test binaries