Reduce allocation to avoid running out of memory on the openbsd/arm builder,
until issue/12032 is resolved.

Update issue #12032

Change-Id: Ibd513829ffdbd0db6cd86a0a5409934336131156
Reviewed-on: https://go-review.googlesource.com/15242Reviewed-by: Dmitry Vyukov <dvyukov@google.com>