[dev.typeparams] runtime: simplify freedefer

Currently, freedefer manually zeros all the fields in the _defer
because simply assigning _defer{} used to cause a nosplit stack
overflow. freedefer is no longer nosplit, so go back to the simpler,
more robust code.

Change-Id: I881f557bab3b1ee7ab29b68e7fb56d0fe6d35d8d
Reviewed-on: https://go-review.googlesource.com/c/go/+/339669
Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>

src/runtime/panic.go

@@ -372,19 +372,7 @@ func freedefer(d *_defer) {
 		unlock(&sched.deferlock)
 	}
 
-	// These lines used to be simply `*d = _defer{}` but that
-	// started causing a nosplit stack overflow via typedmemmove.
-	d.started = false
-	d.openDefer = false
-	d.sp = 0
-	d.pc = 0
-	d.framepc = 0
-	d.varp = 0
-	d.fd = nil
-	// d._panic and d.fn must be nil already.
-	// If not, we would have called freedeferpanic or freedeferfn above,
-	// both of which throw.
-	d.link = nil
+	*d = _defer{}
 
 	pp.deferpool = append(pp.deferpool, d)