runtime: don't always show frames with no function in traceback

If there is no function name, the traceback is generally
    uninformative.  In earlier versions we did not show such frames.
    Restore that behavior.  These frames can be seen with GOTRACEBACK=system.
    
    Reviewed-on: https://go-review.googlesource.com/45431

From-SVN: r249156

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-3b44ad058abda0d1b0b6c928987270da50ab7431
+c4ecdd3edb9febe72b5527481ae3d7310105ca67
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/runtime/traceback_gccgo.go

@@ -94,7 +94,7 @@ func showframe(name string, gp *g) bool {
 	// We want to print those in the traceback.
 	// But unless GOTRACEBACK > 1 (checked below), still skip
 	// internal C functions and cgo-generated functions.
-	if !contains(name, ".") && !hasprefix(name, "__go_") && !hasprefix(name, "_cgo_") {
+	if name != "" && !contains(name, ".") && !hasprefix(name, "__go_") && !hasprefix(name, "_cgo_") {
 		return true
 	}