Fixes #8531.

This issue was introduced by 3fd2814.
Prior to that commit, the `rustdoc_process` function took other branch
of the if statement being modified by this commit. The flag was
previously called `is_host`, and `rustdoc` was run reporting `false`. In
that commit, the flag was changed to `is_rustc_tool`, and rustdoc began
reporting `true`, which resulted in the native directories added by
build scripts no longer being appended to LD_LIBRARY_PATH when running
doctests.

This commit changes the behavior so that we are always appending the
same set of paths, and the only variance is if we are cross compiling or
not. An alternative would be to change rustdoc to always pass
`kind: CompileKind::Host, is_rustc_tool: false`, but as rustdoc is in
fact a rustc tool, that feels wrong. The commit which introduced the bug
did not include context on why the flag was changed to `is_rustc_tool`,
so I don't have a sense for if we should just change that flag to mean
something else.

While the test suite previously had an explicit test for the
library path behavior of `cargo run`, it did not test this for various
test forms. This commit modifies the test to ensure the behavior is
consistent across `run`, normal tests, and doctests.

* add test reproducing #8531
* re-use solution from #10469
* solution fixes test