fix regression in size of nlink_t (broken stat struct) on x86_64

rather than moving nlink_t back to the arch-specific file, I've added
a macro _Reg defined to the canonical type for register-size values on
the arch. this is not the same as _Addr for (not-yet-supported)
32-on-64 pseudo-archs like x32 and mips n32, so a new macro was
needed.