The current StdSizes most closely matches
the gc compiler, and the uses I know of that care
which compiler the sizes are for are all for
the gc compiler, so call the existing
implementation "gc".

Updates #17586
Fixes #19351

Change-Id: I2bdd694518fbe233473896321a1f9758b46ed79b
Reviewed-on: https://go-review.googlesource.com/37666
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>