Merge pull request #798 from aap-sc/aap-sc/mcounteren_fixup

CSR_MCOUNTEREN should not exist if U-mode is not supported

src/target/riscv/riscv.c

@@ -5193,6 +5193,9 @@ int riscv_init_registers(struct target *target)
 				case CSR_VLENB:
 					r->exist = (info->vlenb > 0);
 					break;
+				case CSR_MCOUNTEREN:
+					r->exist = riscv_supports_extension(target, 'U');
+					break;
 			}
 
 			if (!r->exist && !list_empty(&info->expose_csr)) {