runtime: split gc_m into gcMark and gcSweep

runtime: split gc_m into gcMark and gcSweep

This is a nice split but more importantly it provides a better
way to fit the checkmark phase into the sequencing.

Also factor out common span copying into gcSpanCopy.

Change-Id: Ia058644974e4ed4ac3cf4b017a3446eb2284d053
Reviewed-on: https://go-review.googlesource.com/5333
Reviewed-by: Austin Clements <austin@google.com>