Auto merge of #2760 - m-ou-se:netbsd-syscall-h, r=Amanieu

Add netbsd's syscall.h.

This adds https://ftp.netbsd.org/pub/NetBSD/NetBSD-current/src/sys/sys/syscall.h

(I only need `SYS___futex`, but I added the rest as well.)

Author: bors