[RISCV][MC] Prioritize features in MultiMismatchFallback (#215737)
Add the `PrioritizeFeatureInMultipleNearMisses` AsmParser option and
enable it for RISCV.
When an instruction has both missing features and multiple invalid
operands, prioritize the feature entry when populating `MultiMismatchFallback` so
that diagnostics identify the missing extension.
Keep the existing `MultiMismatchFallback` behavior unchanged for targets
that do not enable the option.
This change was assisted by AI.