Use an Arch enum and feature flags instead of cfg(target_arch)

[nbaksalyar]

Instead of using #[cfg(target_arch = "x86_64")] for architecture-specific implementations, introduce corresponding feature flags - e.g.: #[cfg(feature = "x86_64")].

In addition to that, introduce an enum to be used for type arguments:

enum Arch { x86_64 }

This is required for the possibility of future multi-arch builds of Headcrab - for example, in case if we'd want to work with a core dump generated on an ARM-based machine (or connect to that machine remotely using the gdb-serial target).