[fuchsia] Add zx_vmo_replace_as_executable calls
[fuchsia] Add zx_vmo_replace_as_executable calls

In the future, newly created VMOs on Fuchsia will not be mappable as
executable by default. In preparation for that, start using
zx_vmo_replace_as_executable to mark Fuchsia's VMOs as executable as
needed.

SEC-42

TEST=CQ

Change-Id: I9df86bbb33b02042260727d64e74b7146eb1d1a6
Reviewed-on: https://dart-review.googlesource.com/76304
Reviewed-by: Zach Anderson <9f8b31e960a68dade71618f1ff6da6e23a1edfc9@google.com>
Commit-Queue: Zach Anderson <9f8b31e960a68dade71618f1ff6da6e23a1edfc9@google.com>

dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk