RISC-V: verify & fix perf of string comparison

There are different string comparison (UU/LL/UL/LU), and there are 2 implementation, scalar and vector versions. And both implementation are bit complicated, so it's worth to double check both implementations and verify the performance gain of vector version.

There is improvement in some cases, but also bring some regression in some cases, especially for compareToUU.

## tested on K230-CanMV
warmup: 10 times
iteration: 10 times


Benchmark (size) Score+rvv Score-rvv
com.arm.benchmarks.intrinsics.StringCompareToDifferentLength.compareToLL 24 4242936.876 7227607.14 1.703
com.arm.benchmarks.intrinsics.StringCompareToDifferentLength.compareToLL 36 5738695.363 8157070.353 1.421
com.arm.benchmarks.intrinsics.StringCompareToDifferentLength.compareToLL 72 7163243.984 7209568.036 1.006
com.arm.benchmarks.intrinsics.StringCompareToDifferentLength.compareToLL 128 8627566.301 12720927.51 1.474
com.arm.benchmarks.intrinsics.StringCompareToDifferentLength.compareToLL 256 14632020.04 16291127.26 1.113
com.arm.benchmarks.intrinsics.StringCompareToDifferentLength.compareToLL 512 26539410.59 23612505.95 0.89
com.arm.benchmarks.intrinsics.StringCompareToDifferentLength.compareToLU 24 4913490.894 10454585.94 2.128
com.arm.benchmarks.intrinsics.StringCompareToDifferentLength.compareToLU 36 7230036.286 13561865.48 1.876
com.arm.benchmarks.intrinsics.StringCompareToDifferentLength.compareToLU 72 9525418.104 21901656.51 2.299
com.arm.benchmarks.intrinsics.StringCompareToDifferentLength.compareToLU 128 12645301.4 37351484.04 2.954
com.arm.benchmarks.intrinsics.StringCompareToDifferentLength.compareToLU 256 21147886.68 64886475.43 3.068
com.arm.benchmarks.intrinsics.StringCompareToDifferentLength.compareToLU 512 39738017.94 125169103.6 3.15
com.arm.benchmarks.intrinsics.StringCompareToDifferentLength.compareToUL 24 5183884.427 11040441.7 2.13
com.arm.benchmarks.intrinsics.StringCompareToDifferentLength.compareToUL 36 7421224.1 13879329.16 1.87
com.arm.benchmarks.intrinsics.StringCompareToDifferentLength.compareToUL 72 9739241.916 22346979.93 2.295
com.arm.benchmarks.intrinsics.StringCompareToDifferentLength.compareToUL 128 12115336.28 35576652.6 2.936
com.arm.benchmarks.intrinsics.StringCompareToDifferentLength.compareToUL 256 21473923.71 65486723.87 3.05
com.arm.benchmarks.intrinsics.StringCompareToDifferentLength.compareToUL 512 39885299.16 126584284.1 3.174
com.arm.benchmarks.intrinsics.StringCompareToDifferentLength.compareToUU 24 5869568.01 6264293.13 1.067
com.arm.benchmarks.intrinsics.StringCompareToDifferentLength.compareToUU 36 7426068.273 7166385.007 0.965
com.arm.benchmarks.intrinsics.StringCompareToDifferentLength.compareToUU 72 10526248.56 11169251.07 1.061
com.arm.benchmarks.intrinsics.StringCompareToDifferentLength.compareToUU 128 15267275.25 14296156.49 0.936
com.arm.benchmarks.intrinsics.StringCompareToDifferentLength.compareToUU 256 27485452.58 21502833.49 0.782
com.arm.benchmarks.intrinsics.StringCompareToDifferentLength.compareToUU 512 52373404.61 36336863.6 0.694