[RISCV] Add +optimized-nfN-segment-load-store (#114414)

This is a follow up to #111511, where after benchmarking we learnt that
the Banana Pi F3 has fast segmented loads for not just NF=2, but also
NF=3 and NF=4:
https://github.com/preames/bp3-microarch#vlseg_lmul_x_sew_throughput

This adds tuning features to allow these segment loads and stores to be
costed cheaper and enables it for the spacemit-x60.

It also enables +optimized-nf2-segment-load-store by default in the
generic tuning to maintain the previous behaviour when compiled without
-mcpu or -mtune.

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