Need -ffloat-store when compiling e_rem/k_rem.

 * Add CC_REM_PIO2 variable when compiling e_rem_pio2.c and
   k_rem_pio2.c.
 * On linux and freebsd, set CC_REM_PIO2 to -ffloat-store so that
   proper rounding happens.  Not needed on Darwin because Darwin
   always uses sse2 and not x87.