As discussed in person, this adds the code from v8-native-prototype into
V8 proper, guarded by GYP flags that do not build the code by default.
Passing wasm=on to 'make' or setting v8_wasm as a GYP flag activates
building of this code.

An additional header file is added to and exported from the compiler
directory, src/compiler/wasm-compiler.h. This exposes a limited interface
with opaque Node and Graph types to the decoder to build TF graphs, as
well as functions to compile WASM graphs.

The mjsunit tests added are blacklisted because they fail without the
WASM object exposed to JS, which is also disabled by the build config
option.

This corresponds closely to https://github.com/WebAssembly/v8-native-prototype/commit/5981e06ebc9b1e578831d03100f17ebb77970ee0, with some formatting fixes and moving some files into src/compiler.

R=mstarzinger@chromium.org, bradnelson@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1504713014

Cr-Commit-Position: refs/heads/master@{#32794}