Since wasm has no direct notion of foreign globals,
pass the ffi object on to the AsmWasmBuilder
so that foreign globals can be extracted at module
instantiation time.

BUG= https://code.google.com/p/v8/issues/detail?id=4203
TEST=mjsunit/asm-wasm
R=titzer@chromium.org,aseemgarg@chromium.org
LOG=N

Review URL: https://codereview.chromium.org/1677373002

Cr-Commit-Position: refs/heads/master@{#33956}