runtime: mapassign_* should use typedmemmove to update keys

We need to make sure that when the key contains a pointer, we use
a write barrier to update the key.

Also mapdelete_* should use typedmemclr.

Fixes #21297

Change-Id: I63dc90bec1cb909c2c6e08676c9ec853d736cdf8
Reviewed-on: https://go-review.googlesource.com/53414


Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>