runtime: move *task.DeferFrame here

This is a refactor that makes the next commit simpler. Perhaps it should
have been like this from the beginning but I didn't like all the casts.

src/internal/task/task.go

@@ -23,19 +23,7 @@ type Task struct {
 
 	// DeferFrame stores a pointer to the (stack allocated) defer frame of the
 	// goroutine that is used for the recover builtin.
-	DeferFrame *DeferFrame
-}
-
-// DeferFrame is a stack allocated object that stores information for the
-// current "defer frame", which is used in functions that use the `defer`
-// keyword.
-// The compiler knows the JumpPC struct offset.
-type DeferFrame struct {
-	JumpSP     unsafe.Pointer // stack pointer to return to
-	JumpPC     unsafe.Pointer // pc to return to
-	Previous   *DeferFrame    // previous recover buffer pointer
-	Panicking  bool           // true iff this defer frame is panicking
-	PanicValue interface{}    // panic value, might be nil for panic(nil) for example
+	DeferFrame unsafe.Pointer
 }
 
 // getGoroutineStackSize is a compiler intrinsic that returns the stack size for