[wasm] Use the C_WASM_ENTRY code type for cctest GetWrapperCode()

It appears in the cctest context, both the old STUB, and the current 
WASM_FUNCTION "just work"; however, in the upcoming off-the-gc wasm 
world, we codegen call sites differently on x64 - far calls for 
js-to-wasm (this case), and near calls otherwise.

Bug: 
Change-Id: Iebf8acf164f07742fc367b7bbf266913dbc60c46
Reviewed-on: https://chromium-review.googlesource.com/735131Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Mircea Trofin <mtrofin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48889}

test/cctest/wasm/wasm-run-utils.cc

@@ -353,7 +353,7 @@ Handle<Code> WasmFunctionWrapper::GetWrapperCode() {
     }
 
     CompilationInfo info(ArrayVector("testing"), isolate, graph()->zone(),
-                         Code::WASM_FUNCTION);
+                         Code::C_WASM_ENTRY);
     code_ = compiler::Pipeline::GenerateCodeForTesting(&info, descriptor,
                                                        graph(), nullptr);
     CHECK(!code_.is_null());