Auto merge of #3145 - alexcrichton:rustdoc-cross-test, r=brson

Test requested --target from source of truth

We skip doc tests for any cross compiles (as they don't work) but to detect a
cross compile we checked `--target` but forgot to check other locations like
`CARGO_BUILD_TARGET` or `[build.target]`. This alters the check to ensure that
it verifies from the source of truth whether a cross compilation happened or
not.

Closes #3143