This is a band-aid, but it fixes the problem
until a deeper fix is in place.

Testing with genpkg -n 50000, I see:

Before:

      154.67 real       184.66 user         3.15 sys

After:

       61.82 real        96.99 user         2.17 sys


Fixes #14781.

Change-Id: I24c7822d60c289bdd6a18a7840b984954c95f7d4
Reviewed-on: https://go-review.googlesource.com/20696Reviewed-by: Robert Griesemer <gri@golang.org>