Add 32-bit test

[newpavlov]

Small addition to #874. It looks like musl is faster to download and install, so I've chose it over the GNU target.

[dhardy]

I believe mips-unknown-linux-gnu is 32-bit (there is also mips64-unknown-linux-gnuabi64). Is there any other reason to add this?

Since nearly all the builders are quite fast (ironically, disabling caching helped a lot), adding another builder isn't a problem (if it's useful).

[newpavlov]

Well, in theory it's possible to create bugs which will break code only for 32-bit LE target or 64-bit BE one. But I guess probability of doing so accidentally is low enough, so we can ignore it.

On the other hand 32-bit LE targets are probably relatively popular (wasm32 being one of them), so it may be worth to run tests on at least one of them.

Feel free to close this PR if you don't think that such job is useful enough.

[dhardy]

Fair argument; there may be some benefit and there's not a big cost to adding this.