Auto merge of #40382 - alexcrichton:split-tested-targets, r=brson

Auto merge of #40382 - alexcrichton:split-tested-targets, r=brson

travis: Split the linux-tested-targets builder

Travis only gives us 30GB disk space and we don't currently have an option to
increase that. Each musl target generates "hello world" binaries of about 3.5MB
in size, and we're testing two targets in the same image. We have around 3k
run-pass tests and 2 musl targets which works out to around 20GB. That's
dangerously close to the limit and is causing PRs to bounce.

This PR splits up the builder in two, one for x86_64 musl and the other for
i686. Hopefully that'll keep us under the disk limit.

Closes #40359