The workthegc function was being inlined, and the slice did not
escape, so there was no memory allocation. Use a sink variable to
force memory allocation, at least for now.

Fixes #23343

Change-Id: I02f4618e343c8b6cb552cb4e9f272e112785f7cf
Reviewed-on: https://go-review.googlesource.com/122576
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>