fix mobile.sh build (#26975)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/26975
ExportModule doesn't exist in mobile libtorch.a, it doesn't fail for
regular mobile build guess _save_for_mobile was stripped altogether.
But for host toolchain with different linker flag this will fail.
Add #if macro as Module::save.
Test Plan: - scripts/build_mobile.sh works;
Differential Revision: D17629869
Pulled By: ljk53
fbshipit-source-id: 7d3cebe0a7c3f7b56928eb5a9d9c9174403fe6e5
