[vm, ffi] Emit the FFI callback stub as a raw blob and place it in the Fuchsia package.

Any blob in the package can be loaded as an executable VMO.

TEST=child CL
Bug: https://dartbug.com/52579
Change-Id: I61f7a84453da4da4733c5a70d4d19f0a6652581b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/496661
Commit-Queue: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Alexander Aprelev <aam@google.com>

runtime/tests/vm/dart/exported_symbols_test.dart

@@ -366,6 +366,7 @@ main() {
     "Dart_TypeToNullableType",
     "Dart_TypeVoid",
     "Dart_VersionString",
+    "Dart_WriteCallbackStub",
     "Dart_WriteHeapSnapshot",
     "Dart_WriteProfileToTimeline",
     if (isAOTRuntime) ...[