cpu/cortexm: clear FPU state in cpu_switch_context_exit()

[kaspar030]

Contribution description

When an exiting thread that was using the FPU would cause a context switch to a non-FPU-using thread, the lazy stacking could stay activated with outdated info, causing memory corruption:

1. fpu using thread exits, lands in sched_task_exit
2. sched_active_thread is set to NULL
3. cpu_switch_context_exit triggers pendsv
4. pendsv exception entry creates an extended stack frame, configures lazy stacking with FPCAR pointing to the exiting thread's reserved space
5. isr_pendsv skips storing the thread context because sched_active_thread is NULL
6. isr_pendsv, when jumping to a thread that didn't use fpu (thus the restored EXC_RETURN[4] is set), *doesn't restore FPU context
7. note that lazy stacking was activated due to the thread using FPU, but in ISR, no fpu is used, and the VSTR/VLDR were skipped
8. next time any fpu is used, the lazy stacking takes place -> the FPU registers get stored to the old thread's reserved space -> memory corruption

Testing procedure

The new test in #18641 triggers this.

Issues/PRs references

#18641

Alternative to #18691.

[riot-ci]

Murdock results

✔️ PASSED

5cc43e5 cpu/cortexm: clear FPU state in cpu_switch_context_exit()

Success	Failures	Total	Runtime
1981	0	1981	06m:53s

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