Redundant F vs Zfinx encoding

[jrtc27]

Currently we have both sys_enable_fdext and sys_enable_zfinx, and assert the nonsense fourth combination of F+Zfinx isn't the case in: https://github.com/riscv/sail-riscv/blob/085cfcd3fc3be92aaa86b4caabf9d248eae5f83e/model/riscv_sys_control.sail#L562-L563

It might be cleaner to instead have this as a true tristate with no redundant encoding.