Make OPERANDS_PER_INSTR part of build_config

[cathales]

This parameter was used in one file; it is used in several files since https://github.com/openhwgroup/cva6/pull/2395

We  should:

• Add an unsigned int OperandsPerInstr field to cva6_cfg_t (but not cva6_user_cfg_t)
• Calculate it in build_config as it is currently calculated in issue_read_operands
• Use it everywhere it is possible, instead of the equivalent expression.