runtime: use explicit NOFRAME on openbsd/amd64

This CL marks some openbsd assembly functions as NOFRAME to avoid
relying on the implicit amd64 NOFRAME heuristic, where NOSPLIT functions
without stack were also marked as NOFRAME.

Updates #58378

Change-Id: I993549df41a93255fb714357443f8b24c3dfb0a4
Reviewed-on: https://go-review.googlesource.com/c/go/+/466455
Reviewed-by: Keith Randall <[email protected]>
Run-TryBot: Quim Muntal <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
Reviewed-by: Joel Sing <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>

Author: qmuntal

src/cmd/internal/obj/x86/obj6.go

@@ -614,7 +614,7 @@ func preprocess(ctxt *obj.Link, cursym *obj.LSym, newprog obj.ProgAlloc) {
 	var usefpheuristic bool
 	switch ctxt.Headtype {
 	case objabi.Hwindows, objabi.Hdarwin, objabi.Hlinux, objabi.Hdragonfly,
-		objabi.Hfreebsd, objabi.Hnetbsd:
+		objabi.Hfreebsd, objabi.Hnetbsd, objabi.Hopenbsd:
 	default:
 		usefpheuristic = true
 	}