Run CircleCi coverage for 32-bit too

[wilzbach]

See also: dlang/dmd#7718

[dlang-bot]

Thanks for your pull request, @wilzbach!

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[wilzbach]

Jenkins failure is unrelated and should be gone soon (-> dlang/ci#132)

[wilzbach]

75.411% (+1.001%) compared to 2d45fcb

It has a measurable effect 👻

[andralex]

cc @MartinNowak

[wilzbach]

We initially planned to run the coverage on 32-bit too (hence there were no changes to .circleci/run.sh needed).
However, for CircleCi 1.0 the parallelism was always set to 1. A couple of weeks ago we switched to CircleCi 2.0 and thus the switch to parallelism: 2 is now possible.
The druntime test suite runs very quickly (~2 mins) + there aren't that many PRs compared to DMD or Phobos, so we don't have to worry about this here.
FWIW we run CircleCi with parallelism: 3 at dlang/dmd and the test take ~18 mins to run there, so if we ever run into a bottleneck it's at dmd.