runtime: "non in-use span in unswept list" flake on darwin-amd64

https://go-review.googlesource.com/c/go/+/81775 flaked on darwin-amd64 due to:

    # cmd/compile (testmain)
    runtime: bad span s.state=0 s.sweepgen=564617216 sweepgen=12
    fatal error: non in-use span in unswept list

https://storage.googleapis.com/go-build-log/5cf4a738/darwin-amd64-10_11_e7ffe203.log

/cc @aclements 