[RISCV] Add cost model for fma (#126076)
This change builds on PR #125683, which added a cost model for fmuladd.
To ensure completeness, this patch extends the cost model to also cover fma, using the same costing approach as fmuladd.
I plan to send a follow-up patch that includes the cost model vp_fma and vp_fmuladd, and their tests.
