Rather than bake foreign globals into the module
at compile time, add a __foreign_init__ function that can be called
after instantiation with foreign values gathers using keys
in a separately generated FixedArray.

This is an incremental step towards being able to enable asm->wasm
on for general traffic, behind a flag.

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

Review-Url: https://codereview.chromium.org/1999523002
Cr-Commit-Position: refs/heads/master@{#36418}