When deleting entries from a map, only clear the key and value
if they contain pointers. And use memclrHasPointers to do so.

While we're here, specialize key clearing in mapdelete_faststr,
and fix another missed usage of add in mapdelete.

Benchmarking impeded by #21546.

Change-Id: I3f6f924f738d6b899b722d6438e9e63f52359b84
Reviewed-on: https://go-review.googlesource.com/57630
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>