Tune generic SVE parameters closer to other SVE cores and add tunings to baseline AArch64

[Mousius]

Previously baseline AArch64 was left without SWITCH_RATIO or GEMM_PREFERED_SIZE and older default values, but it can be seen across other cores that these values seem to work for many devices.

This should allow us to use the ARMV8 and ARMV8SVE targets in a minimal build without losing too much performance.