psa_crypto_sys is not no_std

[chrysn]

Starting to look into how this could be used on RIOT OS (that since its 2023.10 release has PSA support on its C side), I've noticed that psa_crypto_sys is no_std.

Is that an oversight (if so, it may be convenient to test that things are buildable on no_std, eg. by including a build with --target , similar to the CI commits of wasmi-labs/wasmi#387); if not: is there a plan for how this is to be used on embedded? (Indeed it might be that for RIOT's purposes the psa_crypto_sys crate is patched to go through a different header discovery route, though I'd hope that this can be done upstream by just providing an environment variable).