Fixes some ragg2 x64 assembly generation issues

Enables ragg2 to use elf64 as an output format.
Fixes the syscall registers and syscall generation(?).
Fixes an issue where an 8B address being added to rbp was only added 4B above something else and overwriting the data when emitting a string.

There's still some weirdness going on with some loops and pushing 4B values then popping 8B, but this at least makes simple scripts without branching work.