MIPS: math-emu: Move various objects into an ar library.

ieee754d.o contains only debug code and dp_sqrt.o and sp_sqrt.o contain
code which for MIPS I/II/III systems we don't want to link.  Again the
savings can be considerable for some systems:

$ mips-linux-size --totals ieee754d.o dp_sqrt.o sp_sqrt.o
   text	   data	    bss	    dec	    hex	filename
   1624	      0	      0	   1624	    658	ieee754d.o
   2016	      0	      0	   2016	    7e0	dp_sqrt.o
    736	      0	      0	    736	    2e0	sp_sqrt.o
   4376	      0	      0	   4376	   1118	(TOTALS)

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>