x/build: add openbsd/arm builder

[FiloSottile]

@4a6f656c used to maintain one but it doesn't seem to be running anymore.

Cloud arm32 is hard to find, so this might end up being a Raspberry Pi on my desk.

[4a6f656c]

I've resurrected the openbsd/arm builder and should be able to continue running it for the foreseeable future (or until another option is found).

[bcmills]

I see that the builder is running, but it seems not to have enough memory to complete the tests reliably. Basically all of the runs are failing with fatal error: runtime: out of memory.

Is it possible to add more RAM to the builder, or perhaps configure it with some swap to deal with bigger tests?

Examples:
https://build.golang.org/log/17984e5b3a059c1882855a2c8774fdb7d8ecfad6
https://build.golang.org/log/5233126dc3a18b78a2fee33288f378485eeb66ac
https://build.golang.org/log/ce51b2eebed1b20dc6be028224d613fda8bcc216

[4a6f656c]

@bcmills - the platform is limited to 1GB of data size and this was a regression from Go 1.12, which was fixed via issue #27739.

[bcmills]

@4a6f656c, please disable the openbsd-arm builder until it can be converted to use cmd/buildlet.

The old-style builders are no longer supported (#21191 (comment)), and we can't easily fix configuration issues with them (such as #32838).

[4a6f656c]

@bcmills @dmitshur - a replacement openbsd-arm builder (host-openbsd-arm-joelsing) is running with cmd/buildlet, however is repeatedly failing in the coordinator (server side) with a writeSnapshot: failed to write snapshot to GCS timeout... I presume that my somewhat limited upstream bandwidth, combined with being a long way away (network wise), means that the time limit is insufficient to complete the upload. Could you please investigate?

[bcmills]

(CC @toothrot, @cagedmantis)

[4a6f656c]

@bcmills @toothrot @cagedmantis - I've turned the buildlet off since it has just been spinning its wheels for the last month. Please let me know when I should try again.