[vm] Fix msvc x64/arm64 cross-build

This is a follow-up to
https://dart-review.googlesource.com/c/sdk/+/493921.

TEST=ci

Change-Id: I136845bc6b2ae0582100b8acadd9be0015d7694c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/494022
Reviewed-by: Alexander Aprelev <aam@google.com>
Commit-Queue: Alexander Markov <alexmarkov@google.com>

runtime/vm/simulator_arm64.cc

@@ -1829,6 +1829,8 @@ struct CallbackContext {
   uword sp;
 };
 
+#if defined(SIMULATOR_FFI) && defined(HOST_ARCH_ARM64)
+
 extern "C" void DoRedirectedFfiCallback(CallbackContext* ctxt,
                                         uword trampoline) {
   // Assumptions in ffi_trampolines_arm64.S
@@ -1858,6 +1860,8 @@ extern "C" void DoRedirectedFfiCallback(CallbackContext* ctxt,
   sim->DoRedirectedFfiCallback(thread, ctxt, &out);
 }
 
+#endif  // defined(SIMULATOR_FFI) && defined(HOST_ARCH_ARM64)
+
 // Compare FfiCallbackTrampolineStub.
 void Simulator::DoRedirectedFfiCallback(Thread* thread,
                                         CallbackContext* ctxt,