Also, add a runtime function to call the interpreter, passing a
stack-allocated buffer holding the arguments.
The WASM_INTERPRETER_ENTRY stub allocates the stack slot for the
arguments, fills it, and calls to the wasm interpreter.
It's abi is compatible with WASM functions, such that we can just
replace a call to a WASM_FUNCTION with a call to
WASM_INTERPRETER_ENTRY.
See tracking bug to get the overall picture.

BUG=v8:5822
R=titzer@chromium.org

Review-Url: https://codereview.chromium.org/2619803004
Cr-Commit-Position: refs/heads/master@{#42242}