runtime: in traceback, print panic rather than runtime.gopanic

The gc toolchain does the same thing, in gentraceback in
    runtime/traceback.go.
    
    The test for this is TestPanicTraceback in runtime/crash_test.go.  We
    don't yet run that test, but we will in a future change.
    
    Reviewed-on: https://go-review.googlesource.com/46397

From-SVN: r249495

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-075e67bdbcb730669c1af1aa2d53bb77cbb2a3c5
+f70ef19badafb85b1caa72b51b0082deb48d433a
 
 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

@@ -77,7 +77,11 @@ func traceback(skip int32) {
 func printtrace(locbuf []location, gp *g) {
 	for i := range locbuf {
 		if showframe(locbuf[i].function, gp) {
-			print(locbuf[i].function, "\n\t", locbuf[i].filename, ":", locbuf[i].lineno, "\n")
+			name := locbuf[i].function
+			if name == "runtime.gopanic" {
+				name = "panic"
+			}
+			print(name, "\n\t", locbuf[i].filename, ":", locbuf[i].lineno, "\n")
 		}
 	}
 }