Rearrange target platforms, add memset benchmarks, improve memset

Rearrange the target platforms corresponding to different armv7
platforms.

Add memset benchmarks/validation to the benchmark program.

Add NEON support for memset, both for Thumb2 and non-Thumb2 mode.
At least on a Cortex A8, it is not faster than an optimized
memset not using NEON.