Linux 5.16 compat: don't use XSTATE_XSAVE to save FPU state

Linux 5.16 moved XSTATE_XSAVE and XSTATE_XRESTORE out of our reach,
so add our own XSAVE{,OPT,S} code and use it for Linux 5.16.

Please note that this differs from previous behavior in that it
won't handle exceptions created by XSAVE an XRSTOR. This is sensible
for three reasons.

• Exceptions during XSAVE and XRSTOR can only occur if the feature is not supported or enabled or the memory operand isn't aligned on a 64 byte boundary. If this happens something else went terribly wrong, and it may be better to stop execution.
• Previously we just printed a warning and didn't handle the fault, this is arguable for the above reason.
• All other *SAVE instruction also don't handle exceptions, so this at least aligns behavior.

Finally add a test to catch such a regression in the future.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Attila Fülöp <attila@fueloep.org>
Closes #13042
Closes #13059