reduce syscall wrapper dependences in DR

Today, a tiny change in a syscall wrapper sequence breaks DR. This issue
covers better future-proofing DR by improving the places where we assume
particulars about syscall wrappers:
- syscalls_init() looks at precise bytes in the wrapper to identify the
syscall gateway
- syscall_wrapper_ilist() and clean_syscall_wrapper() rely on being able to
construct a precise syscall wrapper from scratch in order to handle
hooks, including our own native_exec hooks
- the winsysnums tool assumes things like no branches (except a few
hardcoded cases) in its attempt to identify all system calls in an
arbitrary system library

Xref #1598