Move compilation target from concordium_base/cargo.toml to Setup.hs

[soerenbf]

Purpose

To avoid conflicts when compiling dependants on concordium_base, this proposal moves the compilation target specification from the concordium_base crate to the compilation done by the haskell dependant.

One instance where a conflict occurs is, if concordium_base is used as both a dependency in build.rs and src/*.rs. In ./mobile_wallet this was worked around by aligning the profile used for build.rs with the one used for src/lib.rs, however this was not a feasible solution in another project and is hard for other developers to debug if they use this in a similar fashion.

I don't know if this should be marked as a change in the changelog or not?

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.
• (If necessary) I have updated the CHANGELOG.