libsys: add a dependency for syscall stubs

Make syscall stubs generated by lib/libsys/Makefile.sys depend on it.
This will have some false positives, but generating and assembling them
is fast. Also add slightly dubious dependencies on compat.h and SYS.h.

While here, fix the comment documenting the assembly origin.

Reviewed by: kib, emaste
Differential Revision: https://reviews.freebsd.org/D51671