cmd/link, runtime: mark goexit as the top of the call stack

This CL adds a new attribute, TOPFRAME, which can be used to mark
functions that should be treated as being at the top of the call
stack. The function `runtime.goexit` has been marked this way on
architectures that use a link register.

This will stop programs that use DWARF to unwind the call stack
from unwinding past `runtime.goexit` on architectures that use a
link register. For example, it eliminates "corrupt stack?"
warnings when generating a backtrace that hits `runtime.goexit`
in GDB on s390x.

Similar code should be added for non-link-register architectures
(i.e. amd64, 386). They mark the top of the call stack slightly
differently to link register architectures so I haven't added
that code (they need to mark "rip" as undefined).

Fixes #24385.

Change-Id: I15b4c69ac75b491daa0acf0d981cb80eb06488de
Reviewed-on: https://go-review.googlesource.com/c/go/+/169726
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/internal/obj/objfile.go

@@ -230,6 +230,9 @@ func (w *objWriter) writeSymDebug(s *LSym) {
 	if s.NoSplit() {
 		fmt.Fprintf(ctxt.Bso, "nosplit ")
 	}
+	if s.TopFrame() {
+		fmt.Fprintf(ctxt.Bso, "topframe ")
+	}
 	fmt.Fprintf(ctxt.Bso, "size=%d", s.Size)
 	if s.Type == objabi.STEXT {
 		fmt.Fprintf(ctxt.Bso, " args=%#x locals=%#x", uint64(s.Func.Args), uint64(s.Func.Locals))
@@ -342,6 +345,9 @@ func (w *objWriter) writeSym(s *LSym) {
 	if ctxt.Flag_shared {
 		flags |= 1 << 3
 	}
+	if s.TopFrame() {
+		flags |= 1 << 4
+	}
 	w.writeInt(flags)
 	w.writeInt(int64(len(s.Func.Autom)))
 	for _, a := range s.Func.Autom {