Add support for emitting code with a single return at the end. (#153)

This also enables testing of the wasmtests tests.

This also updates for wabt updating to the official "wat" filename
extension, as opposed to "wast".

cranelift/wasmtests/arith.wat

@@ -0,0 +1,13 @@
+(module
+  (memory 1)
+  (func $main (local i32)
+      (set_local 0 (i32.sub (i32.const 4) (i32.const 4)))
+      (if
+          (get_local 0)
+          (then unreachable)
+          (else (drop (i32.mul (i32.const 6) (get_local 0))))
+       )
+  )
+  (start $main)
+  (data (i32.const 0) "abcdefgh")
+)