Allow cross-compiling doctests
This PR allows doctest to receive a --runtool argument, as well as possibly many --runtool-arg arguments, which are then used to run cross compiled doctests.
Also, functionality has been added to rustdoc to allow it to skip testing doctests on a per-target basis, in the same way that compiletest does it. For example, tagging the doctest with "ignore-sgx" disables testing on any targets that contain "sgx". A plain "ignore" still skips testing on all targets.
See [here](https://github.com/rust-lang/cargo/pull/6892) for the companion PR in the cargo project that extends functionality in Cargo so that it passes the appropriate parameters to rustdoc when cross compiling and testing doctests.
Part of [#6460](https://github.com/rust-lang/cargo/issues/6460)
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