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?).