Impact: performance optimization

I did some rebenchmarking with modern compilers and dropping
-funroll-loops makes the function consistently go faster by a few
percent.  So drop that flag.

Thanks to Richard Guenther for a hint.
Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>