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>
diff --git a/arch/riscv/lib/interrupts.c b/arch/riscv/lib/interrupts.c
index d0d8de5..903a1c4 100644
--- a/arch/riscv/lib/interrupts.c
+++ b/arch/riscv/lib/interrupts.c
@@ -87,4 +87,6 @@
 	} else {
 		printf("Reserved\n");
 	}
+
+	hang();
 }