runtime: use debugSelect flag to toggle debug code

This block of code once was commented by the original author, but commenting
code looks a little annoying. However, the debugSelect flag is just for the
situation that debug code will be compiled when debuging, when release this
code will be eliminated by the compiler.

Change-Id: I7b94297e368b515116ef44a36058214ddddf9adb
Reviewed-on: https://go-review.googlesource.com/113395


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>