runtime: don't put container symbols in functab

Container symbols shouldn't be considered as functions in the functab.
Having them present probably messes up function lookup, as you might get
the descriptor of the container instead of the descriptor of the actual
function on the stack.  It also messed up the findfunctab because these
entries caused off-by-one errors in how functab entries were counted.

Normal code is not affected - it only changes (& hopefully fixes) the
behavior for libraries linked as a unit, like:
  net
  runtime/cgo
  runtime/race

Fixes #9804

Change-Id: I81e036e897571ac96567d59e1f1d7f058ca75e85
Reviewed-on: https://go-review.googlesource.com/4290


Reviewed-by: Russ Cox <rsc@golang.org>