target-xtensa: return ENOSYS for unimplemented simcalls

This prevents guest from proceeding with uninitialised garbage returned
from unimplemented simcalls.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
(cherry picked from commit e7eee62a90)

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

xtensa-semi.c

@@ -219,6 +219,8 @@ void HELPER(simcall)(CPUXtensaState *env)
 
     default:
         qemu_log("%s(%d): not implemented\n", __func__, regs[2]);
+        regs[2] = -1;
+        regs[3] = ENOSYS;
         break;
     }
 }