Go runtime support for dragonfly/amd64, largely based of the existing
FreeBSD runtime (with some clues from the varialus/godfly work).
R=bradfitz
CC=golang-dev
https://golang.org/cl/13088044
Split stack checks (morestack) corrupt g->sched,
but g->sched must be preserved consistent for GC/traceback.
The change implements runtime.notetsleepg function,
which does entersyscall/exitsyscall and is carefully arranged
to not call any split functions in between.
R=rsc
CC=golang-dev
https://golang.org/cl/11575044