This change includes a number of improvements since the last WASM
CL. It compiles against a newer version of V8 that matches the current
WASM binary format. Many more WASM instructions are supported, as well
as global variable initializers.

There is also the beginning of a runtime library that implements some
system calls required by the WASM C library. The c2wasm-exe.sh script
can be used to compile a C program to a .wasm module, which is then
compiled by Subzero into a native executable.

This change includes a new Breakpoint instruction, which inserts an
unconditional breakpoint into the executable. This has been helpful in
debugging code generation for some WASM instructions. The Breakpoint
instruction is only completely implemented on X86.

BUG= https://bugs.chromium.org/p/nativeclient/issues/detail?id=4369
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/1876413002 .