A32: Implement Thumb-1 variant of SEVL

While we're at it, also add the Thumb-2 encoding to the encoding table
to make sure it isn't forgotten about in the future.

src/frontend/A32/disassembler/disassembler_thumb.cpp

@@ -254,6 +254,10 @@ public:
         return "sev";
     }
 
+    std::string thumb16_SEVL() {
+        return "sevl";
+    }
+
     std::string thumb16_WFE() {
         return "wfe";
     }