The previous fix (http://crrev.com/2720813002) was problematic for
functions containing i64 parameters or returns. Those would throw a
TypeError when called via WASM_TO_JS and JS_TO_WASM.

Instead, we now unwrap the WASM_INTERPRETER_ENTRY function and call it
directly. This removes the option to later redirect back to the
original wasm function, but this functionality is not needed currently.

Plus drive-by fix to put functions in anonymous namespace in
wasm-compiler.cc.

R=ahaas@chromium.org, titzer@chromium.org
BUG=v8:5971, v8:5822

Review-Url: https://codereview.chromium.org/2725333002
Cr-Commit-Position: refs/heads/master@{#43603}