Remove CS_MODE_N64

- This mode is for the so-called MIPS "N64" ABI; it has nothing to do with the Nintendo 64 game platform.
- N64, O64, et al. are just different ABIs for the 64-bit MIPS architecture, so we replace CS_MODE_N64 with the existing CS_MODE_64

@aquynh Here is the pull request we discussed in private email correspondence.