Optimize strcmp for powerpc64.
Data is loaded by double words and cmpb intruction is used to find '\0'.
Some performance gain rates between the current and the optimized solution:
| String size (Bytes) | <= 16 | 32 | 64 | 128 | 256 | 512 | 1024
| ----- | ----- | ----- | ----- | ----- | ----- | ----- | -----
| Gain rate | -0.81 % | 1.21 % | 3.32 % | 7.52 % | 14.86 % | 27.18 % | 42.08 %