EC-ISA: provide a 10% faster simple parity operation for (k, m=1)

The ISA EC plugin implements a fast region XOR function giving slightly better performance than computing simple parity via GF multiplication. This might be useful for LRC computations.
```
#ISA 
./ceph_erasure_code_benchmark -p isa -P technique=reed_sol_van -i 10000 -s 1000000 -P k=4 -P m=1
#JERAUSRE
./ceph_erasure_code_benchmark -p jerasure -P technique=reed_sol_van -i 10000 -s 1000000 -P k=4 -P 

ISA encoding (4,1) : 3.8 s
JERASURE (4,1): 3.5s
ISA optimized (4,1): 3.15s
```

For 4K en-/decoding the measured effect is smaller:
```
# ./ceph_erasure_code_benchmark -p isa -P technique=reed_sol_van -i 1 -s 4096 -P k=4 -P m=1 
0.000020	4
# ./ceph_erasure_code_benchmark -p jerasure -P technique=reed_sol_van -i 1 -s 4096 -P k=4 -P m=1 
0.000029	4
```