Add host dependency path via -L for cargo_test.

Add host dependency path via -L for cargo_test.

Proc-macro crates' dependencies in the host dependency directory cannot
be found when running `cargo test` with the `--target {target}` flag
set. This adds the host dependency directory with -L when building tests
and a test case that fails before and passes after this patch.

A new function find_host_deps(..) is added to accomplish this which can
determine the path of the host dependencies directory from within
run_doc_tests(..) where the missing library search path is needed.

Fixes #4224
Fixes #4254