[llvm] [RISCV] Avoid let statements in RISCVSystemOperands. NFC (PR #207890)

[Sam Elliott via llvm-commits]

================
@@ -17,7 +17,7 @@ include "llvm/TableGen/SearchableTable.td"
 // CSR (control and status register read/write) instruction options.
 //===----------------------------------------------------------------------===//
 
-class SysReg<string name, bits<12> op> {
+class SysReg<string name, bits<12> op, bit RV32Only = 0> {
----------------
lenary wrote:

```suggestion
class SysReg<string name, bits<12> op, bit RV32Only = false> {
```

And everywhere else  to `true`/`false` please. 

https://github.com/llvm/llvm-project/pull/207890