Add support for syscalls on openbsd amd64. This is based on the
existing freebsd amd64 implementation.
R=mikioh.mikioh, rsc, yourcomputerpal
CC=golang-dev
https://golang.org/cl/4798060
* tweak mksyscall*.pl to be more gofmt-compatible.
* add mkall.sh -syscalls option.
* add sys/mman.h constants on OS X
R=r, eds, niemeyer
CC=golang-dev
https://golang.org/cl/4369044