Add relocation-model option to build profiles

[worr]

rustc has an option to change the way symbol relocation happens at compile
time. While the default of pic is often preferred, there are some cases where
a user may need to output an object where the symbols are non-relocatable
(some kernels, for instance).

Expose the relocation-model argument to the user through the profiles stanza
in Cargo.toml.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @wycats (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. The way Github handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[alexcrichton]

Thanks for the PR! Currently, however, the goal of the profiles in Cargo isn't necessarily to reexport every codegen option of the compiler at the top level (also see #1137). Something like relocation-model which is required to build a project has been thought to belong in the custom target JSON instead of in the manifest perhaps.

That being said, this is an open design question of how to reexport various options like these, and for now I'd prefer to avoid adding new options without exploring the design space a bit more. As a result I'm going to close this for now. I think we'll probably want something like an RFC to move forward here rather than design-via-PR, so feel free to reach out to me on IRC! (acrichto)