FRC target support

[connorworley]

This PR adds support for target platform used in the FIRST Robotics Competition.

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[codyps]

Given that the control system has a habit of changing every few years, might make sense to expand the vendor to either include the year of introduction or the name of the platform.

[connorworley]

Agreed, I've added the platform name and year of release.

Or do you mean changing the whole triple? Right now I'm just matching the triple that the NI-provided toolchain uses.

[aidanhs]

Thanks for the PR @connorworley! We'll check in regularly to make sure @arielb1 or another reviewer gets to this soon.

[parched]

I don't think rust is in the habit of adding vendored targets like this. Would adding a neutral armv7-unknown-linux-gnueabi target be sufficient for your needs?

[alexcrichton]

Thanks for the PR @connorworley! As mentioned by @parched though we tend to add more generalized platforms to the compiler before branching into the specialized ones. We don't currently really have a policy one way or the other about this, it's mostly just gut feelings so far.

Out of curiosity is there a more general target that could be added? Or are custom target specs not sufficient for your use case maybe?

[connorworley]

I could get away with using custom target specs, but the ultimate goal is for the high school students I work with to be able to easily deploy rust code for the roboRIO. In that regard, I think getting this target upstreamed would make it the easiest for them.

[arielb1]

@alexcrichton are we going to be including this target? Should this be discussed in one of the teams?

[codyps]

Looks like this is just a specific cpu & features. Given that, it seems like naming it for the cpu selection & fpu options might be more appropriate (there isn't anything really FRC specific about the target, others with similar cpus could use it).

Is the need here just for better optimization compared to using one of the other arm-*-linux-gnueabi targets? More generally: what customizations here are actually required/desired and why are they required/desired?

[connorworley]

So after some more testing I have a better idea of what's going on, and I think I can get away with just using arm-unknown-linux-gnueabi. If it turns out that any customization are actually necessary, I'll send in a new PR for a generic target supporting those customizations.