runtime: do not execute write barrier on newly allocated slice in growslice

The new slice created in growslice is cleared during malloc for
element types containing pointers and therefore can only contain
nil pointers. This change avoids executing write barriers for these
nil pointers by adding and using a special bulkBarrierPreWriteSrcOnly
function that does not enqueue pointers to slots in dst to the write
barrier buffer.

Change-Id: If9b18248bfeeb6a874b0132d19520adea593bfc4
Reviewed-on: https://go-review.googlesource.com/115996


Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>