fix(threads): xtensa: enable saving/restoring float regs

[kaspar030]

Description

... seems like something we want. esp-wifi also enables this.

Issues/PRs references

Open Questions

Change checklist

• I have cleaned up my commit history and squashed fixup commits.
• I have followed the Coding Conventions.
• I have performed a self-review of my own code.
• I have made corresponding changes to the documentation.

[elenaf9]

Does this impact context switching performance? Because now on each context switch the 15 float registers need to be saved/ restored as well.

[kaspar030]

Does this impact context switching performance? Because now on each context switch the 15 float registers need to be saved/ restored as well.

It probably does. But if we don't save them, they're potentially garbled after a context switch ...

[elenaf9]

But if we don't save them, they're potentially garbled after a context switch

esp-hal disables the floating point co-processor when the feature is disabled, and the relevant registers aren't present in the trapframe in that case. So they can't be used. I ran into a panic myself in the past due to that 😄
Once the feature is enabled, the registers are part of the trapframe, and we automatically save them because we always save and switch the whole trapframe.

So IMO users that need floating point operations can just enable it and it will work with our implementation (also thanks to 0e84775). No need for us to enable it?