ABI hooking + registers fix.

Currently only compiles on AMD64.

Fixed:
- Initial hooking per ABI
- Fixed get/set register for non-native ABI on AMD64

Added:
- get_reg and set_reg in arch/<arch>/arch.{c,h}

Changed:
- tracy_set_hook to require abi (not abi_mask, yet)
- hash_syscall to take an abi argument; this is a temporary solution to a
  bigger problem: we need to get rid of ll.{c,h} for this.

Removed:
- tracy_debug_current_pid