x86: Use PentiumPro-optimized partial_csum() on VIA C7

Testing on the OLPC XO-1.5 (VIA C7-M 1000MHz CPU) shows a partial_csum()
speed increase by a factor of 1.5 when we switch to the Pentium-optimized
version.

Signed-off-by: Daniel Drake <dsd@laptop.org>
Cc: dilinger@queued.net
Signed-off-by: Ingo Molnar <mingo@elte.hu>