runtime/debug: print scavenger details with GOGCTRACE=1

[dominikh]

The periodic scavenger prints details regarding garbage collection and returning of
memory to the OS. The code for printing these statistics is actually in the scavenger
itself.

Calling FreeOSMemory() will not go through the periodic scavenger and in turn not output
any debug information. Adding such output would be beneficial for debugging purposes.

[rsc]

Comment 1:

Labels changed: added priority-later, go1.2maybe, removed priority-triage.

Status changed to Accepted.

[dvyukov]

Comment 2:

Owner changed to @dvyukov.

Status changed to Started.

[dvyukov]

Comment 3:

This issue was closed by revision 9f46efc.

Status changed to Fixed.