runtime: track "scannable" bytes of heap

runtime: track "scannable" bytes of heap

This tracks the number of scannable bytes in the allocated heap. That
is, bytes that the garbage collector must scan before reaching the
last pointer field in each object.

This will be used to compute a more robust estimate of the GC scan
work.

Change-Id: I1eecd45ef9cdd65b69d2afb5db5da885c80086bb
Reviewed-on: https://go-review.googlesource.com/9695
Reviewed-by: Russ Cox <rsc@golang.org>