sw:vec: Fix exception handler

Need to return non-zero code and clear stack
Also remove the extra custom3<func3=3>, or the CPU would be halted
prematurely.

Change-Id: Iaf731388cbe13e0033b16f25710a9cc256bfdb27

springbok/crt0.S

diff --git a/springbok/crt0.S b/springbok/crt0.S
index 7e3ba79..f9e46f6 100644
--- a/springbok/crt0.S
+++ b/springbok/crt0.S
@@ -246,13 +246,15 @@
 
 .weak exception_handler
 exception_handler:
-        .word 0x0000307B # finish (encoded as custom3<func3=3>)
         li   t4, 0 # ERROR logging level
-        li   t5, 0 # Return 0
-        la a0, exception_msg
+        la   a0, exception_msg
         csrr t5, mcause
         .word 0x001E50EFB # simprint t4, a0, t5 (encoded as custom3<func3=0>)
         call print_csrs
+        # Store the return value as mcause to be used in _finish
+        mv   a0, t5
+        # Clear the stack
+        la   sp, _stack_ptr
         j _finish
 
 .weak print_csrs