[lld][WebAssembly] Don't apply data relocations at static constructor time

Instead, export `__wasm_apply_data_relocs` and `__wasm_call_ctors`
separately.

This is required since user code in a shared library (such as static
constructors) should not be run until relocations have been applied to
all loaded libraries.

See: https://github.com/emscripten-core/emscripten/issues/17295

Differential Revision: https://reviews.llvm.org/D128515