x/build: add at least one 32-bit longtest builder

[ALTree]

Currently, the only builder that runs ~every test is the linux-amd64-longtest builder. It's linux, and it's 64 bit.

There's already an issue for a different OS longtest builder (windows: #26529); but having a 32-bit longtest builder would also be useful. See for example #29555: the archive/zip test suite was broken on 32-bit archs and nobody noticed because the bad tests are skipped on short so currently they're not run on any builder.

Filing this as a remainder.

[julieqiu]

/cc @bcmills

[bcmills]

CC @bradfitz @dmitshur

[gopherbot]

Change https://golang.org/cl/202479 mentions this issue: dashboard: add windows and 32-bit longtest builders

[gopherbot]

Change https://golang.org/cl/234520 mentions this issue: dashboard: set GO{,HOST}ARCH in linux-386-longtest builder

[gopherbot]

Change https://golang.org/cl/240512 mentions this issue: dashboard: stop linux-386-longtest builds in x/exp and x/mobile