x86: Enhance perf checksum profiling and x86 implementation

[Neil Horman]

Hey all-
	Sorry for the delay here, but it took me a bit to get the perf bits
working to my satisfaction.  As Ingo requested I added do_csum to the perf
benchmarking utility (as part of the mem suite, since it didn't seem right to
create its own suite).  I've also revamped the do_csum routine to do some smart
prefetching, as it yielded slightly better performance over simple prefetching
at a fixed stride:

Without prefetch:
[root@rdma-dev-02 perf]# ./perf bench mem csum -r x86-64-csum -l 1500B -s 512MB
-i 1000000 -c
# Running mem/csum benchmark...
# Copying 1500B Bytes ...

       0.955977 Cycle/Byte

With prefetch:
[root@rdma-dev-02 perf]# ./perf bench mem csum -r x86-64-csum -l 1500B -s 512MB
-i 1000000 -c
# Running mem/csum benchmark...
# Copying 1500B Bytes ...

       0.922540 Cycle/Byte


About a 3% improvement.

Signed-off-by: Neil Horman <nhorman@xxxxxxxxxxxxx>
CC: sebastien.dugue@xxxxxxxx
CC: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CC: Ingo Molnar <mingo@xxxxxxxxxx>
CC: "H. Peter Anvin" <hpa@xxxxxxxxx>
CC: x86@xxxxxxxxxx

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