Auto merge of #3205 - matklad:fix-rustdoc-ld-path, r=alexcrichton

Pass target environment for rustdoc

This should fix #3200, but I am not sure that this is a correct fix, and I need some input to figure this out.

`rustdoc` is invoked in two places, in `cargo_test.rs` and in `cargo_rustc/mod.rs`. Before the refactoring PR, these invocations used different LD_LIBRARY paths. [The one in cargo_rustc](https://github.com/rust-lang/cargo/pull/3198/files#diff-59acd1a3101aebbb591ac7ab51c19d9eR427) used "host" version, while [the one in cargo_test](https://github.com/rust-lang/cargo/blob/a8baa5b8f36e88170c8c56523b6eb72efc2cc55e/src/cargo/ops/cargo_test.rs#L131) used "target" version.

The original PR changed both to "host", this PR switches both to "target". Is this correct, or should we stick with different environments for building documentation and doctests?