[wasm] Add thread-in-wasm flag builtins for x64 target

This was causing trouble with Fuchsia, since mksnapshot was built and run on
Linux which supports trap handlers, while Fuchsia does not yet. This change
causes the external references to match between Fuchsia and Linux.

Bug: chromium:772057
Change-Id: I8e8f3539e3f5c4b798c364101ef2d16b5137f16d
Reviewed-on: https://chromium-review.googlesource.com/706109Reviewed-by: Mircea Trofin <mtrofin@chromium.org>
Commit-Queue: Eric Holk <eholk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48408}

src/external-reference-table.cc

@@ -219,12 +219,12 @@ void ExternalReferenceTable::AddReferences(Isolate* isolate) {
   // If the trap handler is not supported, the optimizer will remove these
   // runtime functions. In this case, the arm simulator will break if we add
   // them to the external reference table.
-  if (V8_TRAP_HANDLER_SUPPORTED) {
-    Add(ExternalReference::wasm_set_thread_in_wasm_flag(isolate).address(),
-        "wasm::set_thread_in_wasm_flag");
-    Add(ExternalReference::wasm_clear_thread_in_wasm_flag(isolate).address(),
-        "wasm::clear_thread_in_wasm_flag");
-  }
+#ifdef V8_TARGET_ARCH_X64
+  Add(ExternalReference::wasm_set_thread_in_wasm_flag(isolate).address(),
+      "wasm::set_thread_in_wasm_flag");
+  Add(ExternalReference::wasm_clear_thread_in_wasm_flag(isolate).address(),
+      "wasm::clear_thread_in_wasm_flag");
+#endif
   Add(ExternalReference::f64_acos_wrapper_function(isolate).address(),
       "f64_acos_wrapper");
   Add(ExternalReference::f64_asin_wrapper_function(isolate).address(),