Marking of span roots can represent a significant fraction of the time
spent in mark termination. Simply traversing the span list takes about
1ms per GB of heap and if there are a large number of finalizers (for
example, for network connections), it may take much longer.

Improve the situation by splitting the span scan into 128 subtasks
that can be executed in parallel and load balanced by the markroots
parallel for. This lets the GC balance this job across the Ps.

A better solution is to do this during concurrent mark, or to improve
it algorithmically, but this is a simple change with a lot of bang for
the buck.

This was suggested by Rhys Hiltner.

Updates #11485.

Change-Id: I8b281adf0ba827064e154a1b6cc32d4d8031c03c
Reviewed-on: https://go-review.googlesource.com/13112Reviewed-by: Keith Randall <khr@golang.org>