go/ssa/interp: add no-op intrinsic for runtime.init().

Recent changes in the runtime caused the interpreter to call 'getg',
and it should never have gotten that far.

Also, delete bodies of "runtime" functions, since they're too magical.
This makes missing intrinsics cause very obvious failures.

LGTM=gri
R=gri
CC=golang-codereviews
https://golang.org/cl/135330043

go/ssa/interp/external.go

@@ -91,6 +91,7 @@ func init() {
 		"runtime.GOMAXPROCS":               ext۰runtime۰GOMAXPROCS,
 		"runtime.Goexit":                   ext۰runtime۰Goexit,
 		"runtime.Gosched":                  ext۰runtime۰Gosched,
+		"runtime.init":                     ext۰runtime۰init,
 		"runtime.NumCPU":                   ext۰runtime۰NumCPU,
 		"runtime.ReadMemStats":             ext۰runtime۰ReadMemStats,
 		"runtime.SetFinalizer":             ext۰runtime۰SetFinalizer,
@@ -344,6 +345,10 @@ func ext۰runtime۰Gosched(fr *frame, args []value) value {
 	return nil
 }
 
+func ext۰runtime۰init(fr *frame, args []value) value {
+	return nil
+}
+
 func ext۰runtime۰NumCPU(fr *frame, args []value) value {
 	return runtime.NumCPU()
 }

go/ssa/interp/interp.go

@@ -670,6 +670,19 @@ func Interpret(mainpkg *ssa.Package, mode Mode, sizes types.Sizes, filename stri
 			sz := sizes.Sizeof(pkg.Object.Scope().Lookup("MemStats").Type())
 			setGlobal(i, pkg, "sizeof_C_MStats", uintptr(sz))
 
+			// Delete the bodies of almost all "runtime" functions since they're magic.
+			// A missing intrinsic leads to a very clear error.
+			for _, mem := range pkg.Members {
+				if fn, ok := mem.(*ssa.Function); ok {
+					switch fn.Name() {
+					case "GOROOT", "gogetenv":
+						// keep
+					default:
+						fn.Blocks = nil
+					}
+				}
+			}
+
 		case "os":
 			Args := []value{filename}
 			for _, s := range args {