syscall: add BPF support for darwin/386, darwin/amd64

note: due to issue 1466 the Msghdr and BpfProgram
struct for src/pkg/syscall/ztypes_darwin_386.go,
src/pkg/syscall/ztypes_darwin_amd64.go had to be
edited after the godefs generation.

R=rsc
CC=golang-dev
https://golang.org/cl/4403042
6 files changed