internal/gclayout: make gclayout values constants

The previous versions calculated at init() prevented `interp` from running
in many cases, increasing compile times due to the increased need to revert
the partially interpreted results and also increasing binary runtime because
fewer optimizations had happened during interp.

src/runtime/slice.go

@@ -51,7 +51,7 @@ func sliceGrow(oldBuf unsafe.Pointer, oldLen, oldCap, newCap, elemSize uintptr)
 	var layout unsafe.Pointer
 	// less type info here; can only go off element size
 	if elemSize < unsafe.Sizeof(uintptr(0)) {
-		layout = gclayout.NoPtrs
+		layout = gclayout.NoPtrs.AsPtr()
 	}
 
 	buf := alloc(newCap*elemSize, layout)