The interpreter was not able to call imported wasm functions (hitting
UNIMPLEMENTED). This CL fixes this by creating a "CWasmEntry", which is
signature-specific. It has JS linkage and receives the wasm code object
to call and a buffer containing all arguments (similar to the
interpreter entry). It loads all arguments from the buffer and calls the
given code object.
The c-wasm-entry code objects are cached per instance, such that we
only create them once per signature.

These wasm entry stubs will also allow us to call back to compiled code
from the interpreter, which we might want to do to reduce the slowdown
of executing wasm for debugging.

R=titzer@chromium.org

Bug: chromium:735792
Change-Id: I7fecec3a7bec62a9de40fff115b684759b12a28b
Reviewed-on: https://chromium-review.googlesource.com/600308
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47195}