Fix and regenerate runtime defs for NetBSD.

Whilst the mcontext struct can be handled across architectures,
the registers are provided as defines that index an array, rather
than as members of the struct. Since these are architecture
dependent, include them via a defs_netbsd_<arch>.go file.

R=golang-dev, m4dh4tt3r, rsc
CC=golang-dev
https://golang.org/cl/6190070