runtime: use explicit NOFRAME on plan9/amd64

This CL marks some plan9 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: Ic8c9ab5c1a0897bebc6c1419ddc903a7492a1b0a
Reviewed-on: https://go-review.googlesource.com/c/go/+/466457
TryBot-Bypass: Quim Muntal <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>

Author: qmuntal

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

@@ -617,7 +617,7 @@ 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.Hopenbsd, objabi.Hsolaris:
+		objabi.Hfreebsd, objabi.Hnetbsd, objabi.Hopenbsd, objabi.Hsolaris, objabi.Hplan9:
 	default:
 		usefpheuristic = true
 	}

src/runtime/sys_plan9_amd64.s

@@ -166,7 +166,7 @@ TEXT runtime·settls(SB),NOSPLIT,$0
 	RET
 
 // void sigtramp(void *ureg, int8 *note)
-TEXT runtime·sigtramp(SB),NOSPLIT,$0
+TEXT runtime·sigtramp(SB),NOSPLIT|NOFRAME,$0
 	get_tls(AX)
 
 	// check that g exists