Define some reference platforms for benchmarking

[gilles-peskine-arm]

We currently don't run any benchmarks on the CI. All of our CI runs on virtualized platforms, so we can't have meaningful benchmarks.

The goal of this issue is to define some platforms that we could use for benchmarking. The hardware should be easily obtainable so that we can expect that motivated contributors could reasonably obtain one. So Raspberry Pi: ok; M1 mac: not so good (expensive, we can't expect everybody to have access to one). This is related to, but separate from, having benchmark platforms available on the CI.

A reference platform should be very precisely specified in order to be reproducible: exact hardware, OS, compiler version, compiler options, …

The first set of reference platforms should probably have:

• One low-end Cortex-M;
• One high-end Cortex-M;
• One high-end Cortex-A;
• Maybe a lower-end Cortex-A.