Add a Uoffset32 immediate operand kind.

WebAssembly memory instructions encode a 32-bit unsigned offset that is
used to compute an effective address.

lib/cretonne/meta/base/immediates.py

@@ -26,6 +26,15 @@ offset32 = ImmediateKind(
         'A 32-bit immediate signed offset.',
         default_member='offset')
 
+#: A 32-bit immediate unsigned offset.
+#:
+#: This is used to represent an immediate address offset in WebAssembly memory
+#: instructions.
+uoffset32 = ImmediateKind(
+        'uoffset32',
+        'A 32-bit immediate unsigned offset.',
+        default_member='offset')
+
 #: A 32-bit immediate floating point operand.
 #:
 #: IEEE 754-2008 binary32 interchange format.