occurs using the SSE math processor. Update comments describing the
handling of the exception status bits in coprocessors control words.
Remove GET_FPU_CW and GET_FPU_SW macros which were used only once.
Prefer to use curpcb to access pcb_save over the longer path of
referencing pcb through the thread structure.
Based on the submission by: Ed Alley <wea llnl gov>
PR: amd64/169927
Reviewed by: bde
MFC after: 3 weeks
bc84db6267