x86: Don't bother copying the FPU state for cpu_copy_thread().

[jhb]

Summary

The copied state never gets used since the FPU INITDONE flags are
always cleared in the new threads causing the first DNA trap to
overwrite the saved state with the appropriate initial state.

[jhb]

So now I'm a bit torn on this due to the language for pthread_create() I found in the other review. If the "floating point environment" includes register values, then what we should be doing here instead is avoid clearing the INITDONE flags in the pcb for cpu_copy_thread().