runtime: copy netpoll code from Go 1.7 runtime

gcc/go/gofrontend/MERGE

libgo/Makefile.am

libgo/Makefile.in

libgo/config.h.in

libgo/configure

libgo/configure.ac

libgo/go/runtime/netpoll.go

libgo/go/runtime/netpoll_epoll.go

libgo/go/runtime/netpoll_kqueue.go

libgo/go/runtime/netpoll_nacl.go

libgo/go/runtime/netpoll_solaris.go

libgo/go/runtime/netpoll_stub.go

libgo/go/runtime/netpoll_windows.go

libgo/go/runtime/stubs.go

libgo/go/runtime/time.go

libgo/mkrsysinfo.sh

libgo/runtime/malloc.h

libgo/runtime/mgc0.c

libgo/runtime/netpoll.goc

libgo/runtime/netpoll_epoll.c