[dev.cc] runtime: generate GOOS- and GOARCH-specific files with go generate

Eventually I'd like almost everything cmd/dist generates
to be done with 'go generate' and checked in, to simplify
the bootstrap process. The only thing cmd/dist really needs
to do is write things like the current experiment info and
the current version.

This is a first step toward that. It replaces the _NaCl etc
constants with generated ones goos_nacl, goos_darwin,
goarch_386, and so on.

LGTM=dave, austin
R=austin, dave, bradfitz
CC=golang-codereviews, iant, r
https://golang.org/cl/174290043

src/runtime/runtime2.go

@@ -45,7 +45,13 @@ const (
 	_Pdead
 )
 
-// XXX inserting below here
+// The next line makes 'go generate' write the zgen_*.go files with
+// per-OS and per-arch information, including constants
+// named goos_$GOOS and goarch_$GOARCH for every
+// known GOOS and GOARCH. The constant is 1 on the
+// current system, 0 otherwise; multiplying by them is
+// useful for defining GOOS- or GOARCH-specific constants.
+//go:generate go run gengoos.go
 
 type mutex struct {
 	// Futex-based impl treats it as uint32 key,
@@ -395,14 +401,6 @@ type itab struct {
 	fun    [0]uintptr
 }
 
-const (
-	// TODO: Generate in cmd/dist.
-	_NaCl    = 0
-	_Windows = 0
-	_Solaris = 0
-	_Plan9   = 0
-)
-
 // Lock-free stack node.
 type lfnode struct {
 	next    *lfnode