Add relocatable pointers for wasm memory references that need to be updated when wasm GrowMemory is used. Code generator changes to accept relocatable constants as immediates.

R=titzer@chromium.org, yangguo@chromium.org, bradnelson@chromium.org

Review URL: https://codereview.chromium.org/1759383003

Cr-Commit-Position: refs/heads/master@{#35182}