added nacl as a platform to autoconf scripts
ported unix64 call switch to nacl assembly

straightforward port to nacl assembly quirks
main points:

- use naclret, naclcall and nacljmp instead of regular ret, call and jmp
- use correct mangling macros for rsp and rbp
- load from correct memory offsets (32-bit offset + r15 value)
- move jump tables to non-executable segment
- split address load functions into (address calculation + address load)
Merge pull request #1 from pkit/nacl_port

NaCL port code