runtime: skip stack barrier copy when there are no pointers

After CL 31455, "go fun(n)" may put "n" to write barrier buffer
when there are no pointers in fun's arguments.

Fixes #29362

Change-Id: Icfa42b8759ce8ad9267dcb3859c626feb6fda381
Reviewed-on: https://go-review.googlesource.com/c/155779Reviewed-by: Keith Randall <khr@golang.org>

src/runtime/proc.go

@@ -3303,11 +3303,13 @@ func newproc1(fn *funcval, argp *uint8, narg int32, callergp *g, callerpc uintpt
 		if writeBarrier.needed && !_g_.m.curg.gcscandone {
 			f := findfunc(fn.fn)
 			stkmap := (*stackmap)(funcdata(f, _FUNCDATA_ArgsPointerMaps))
+			if stkmap.nbit > 0 {
 				// We're in the prologue, so it's always stack map index 0.
 				bv := stackmapdata(stkmap, 0)
 				bulkBarrierBitmap(spArg, spArg, uintptr(narg), 0, bv.bytedata)
 			}
 		}
+	}
 
 	memclrNoHeapPointers(unsafe.Pointer(&newg.sched), unsafe.Sizeof(newg.sched))
 	newg.sched.sp = sp