Add arm cortex-m platform constraints

[nathaniel-brough]

This adds platforms cpu constraints for cortex-m targets;

• Cortex-M0
• Cortex-M1
• Cortex-M4
• Cortex-M7
• Cortex-M23
• Cortex-M33
• Cortex-M35P

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[nathaniel-brough]

@googlebot I signed it!

[google-cla]

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[katre]

What is the intent behind these? What compiler toolchains will need to use this?

[nathaniel-brough]

The use case is for targeting cortex-m microcontrollers. Example of existing toolchain. This PR covers the majority of all arm cortex microcontroller architectures. The intent is to shift the silvergasp/bazel-embedded repo to depend on the upstream "platforms" repository rather than this fork.

Example use case includes cross compiling google pigweed.

[katre]

Thanks for your patience, I have been doing some research on ARM cpus. I think this PR makes sense, but can you add comments to each constraint stating that it's for Cortex-M0, etc?

[nathaniel-brough]

No worries that's an easy change.

[google-cla]

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[katre]

Thanks! Once #22 is in (it's already started) I'll merge this.

[katre]

Okay, #22 is merged, could you update this to master and then I'll merge it?

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[nathaniel-brough]

Done!

[katre]

Thanks, starting the merge process now.