Syscall one-off refactoring

This series implements some of @rocallahan's earlier ideas on architecture-neutral replacements for `SYS_foo` constants.  It handles just about everything, except for some `SYS_socketcall` constants that need some better infrastructure in `syscall_defs.h` (we need to be able to talk about syscalls defined on x86-64 that aren't defined on x86--right now we only go the other direction) and some weird checks in `emufs.cc` that I'm not immediately sure how to architecturialize (sp?).
This is where you introduced the `cpuid` regression. For a `getuid32` syscall `!is_geteuid_syscall(ev.Syscall().number, t->arch())` is true so we take the early exit.

We don't need to check `has_geteuid32` here; if the arch don't have it, then `is_geteuid32_syscall` will simply return false. So we can just write this as
````
  if (!is_geteuid32_syscall(ev.Syscall().number, t->arch()) &&
      !is_geteuid_syscall(ev.Syscall().number, t->arch())) {
````