Templatize syscallname (along with a few other things...)

This started out as a simple templatization of `syscallname`, but got a little carried away eradicating `SYS_[a-z]*` from `record_syscall.cc` and `replay_syscall.cc`.

I don't think there's anything wrong with having the necessary functions in those two files be templates, but I see from grepping for `SYS_` that there's many one-offs elsewhere in the codebase.  Introducing `RR_ARCH_FUNCTION` for all of those seems wrong, but maybe they can all be tied together as a family of templated functions or somesuch...