It takes me several minutes every time I want to find where the linker
writes out the _func structures. Add some comments to make this
easier.

Change-Id: Ic75ce2786ca4b25726babe3c4fe9cd30c85c34e2
Reviewed-on: https://go-review.googlesource.com/34390Reviewed-by: Ian Lance Taylor <iant@golang.org>