riscv: hang on unhandled exceptions

Hang on unhandled exceptions to prevent execution in a faulty state.

Signed-off-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Rick Chen <rick@andestech.com>

arch/riscv/lib/interrupts.c

@@ -87,4 +87,6 @@ static void _exit_trap(ulong code, ulong epc, struct pt_regs *regs)
 	} else {
 		printf("Reserved\n");
 	}
+
+	hang();
 }