GDB: set correct data model for x86_64-nacl
This is required for reading typed values from target memory, in particular, to read correct pointer values.
The change is risky. It confuses a lot of code that uses pointer size to distinguish from x86_64 and i386. Luckily, this code is mostly about native debugging and syscalls, so it is not used for NaCl.
Anyway, the way to check for x86_64 by pointer size is lame, as it mixes architecture properties and data model. We plan to clean this up using some better check.
The change is hacky, as it fixes types of registers values directly. This should be done in xml descriptions instead. However, this requires much more work, and this requires introducing NaCl-specific xml files.