util: Optimize _mesa_roundeven with SSE 4.1.

util: Optimize _mesa_roundeven with SSE 4.1.

The SSE 4.1 ROUND instructions let us implement roundeven directly.
Otherwise we assume that the rounding mode has not been modified (as we
do in the rest of Mesa) and use rint().

glibc uses the ROUND instruction in rint() after a cpuid check. This
patch just lets us inline it directly when we're already building for
SSE 4.1.

Reviewed-by: Carl Worth <cworth@cworth.org>