When last_task_used_math gets the cpu, ST0_CU1 is set during context

switch. All other FP programs have ST0_CU1 cleared in their thread_struct,
so that they incur faults on first touching the FPU, and are made the
FPU owner, aka last_task_used_math. used_math determines whether a
thread has used the FPU before; if not, the FPU needs to be initialized
before it can own the FPU. Note: a FP program that has already been
using the fpu (not neccesarily the owner), must reinit the fpu if
it uses the fpu in a signal handler; a sigreturn restores the
original context, discarding the context of the handler.

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