runtime: convert a few traceback-related functions from proc.c to traceback.go

They were in proc.c mainly because there was no portable
traceback source file. As part of converting them to Go,
move to traceback.go.

In order to get access to the PC of _rt0_go,
rename to runtime.rt0_go.

LGTM=r
R=golang-codereviews, r
CC=dvyukov, golang-codereviews, iant, khr
https://golang.org/cl/139110043

src/cmd/api/goapi.go

@@ -405,7 +405,17 @@ func (w *Walker) parseFile(dir, file string) (*ast.File, error) {
 			" wincallbackcontext struct{};" +
 			" _select struct{}; " +
 			"); " +
-			"const ( cb_max = 2000 )"
+			"const (" +
+			" cb_max = 2000;" +
+			" _Gidle = 1;" +
+			" _Grunnable = 2;" +
+			" _Grunning = 3;" +
+			" _Gsyscall = 4;" +
+			" _Gwaiting = 5;" +
+			" _Gdead = 6;" +
+			" _Genqueue = 7;" +
+			" _Gcopystack = 8;" +
+			")"
 		f, err = parser.ParseFile(fset, filename, src, 0)
 		if err != nil {
 			log.Fatalf("incorrect generated file: %s", err)