Previously, gccheckmark could only be enabled or disabled by calling
runtime.GCcheckmarkenable/GCcheckmarkdisable.  This was a necessary
hack because GODEBUG was broken.

Now that GODEBUG works again, move control over gccheckmark to a
GODEBUG variable and remove these runtime functions.  Currently,
gccheckmark is enabled by default (and will probably remain so for
much of the 1.5 development cycle).

Change-Id: I2bc6f30c21b795264edf7dbb6bd7354b050673ab
Reviewed-on: https://go-review.googlesource.com/2603Reviewed-by: Rick Hudson <rlh@golang.org>