In JavaScript code and stubs, JSSP mirrors the CSP but may be unaligned.
    But in WASM code only CSP is used, like native code, and it must be
    aligned.
    Calls into WASM from JS need to carefully align the C stack
    pointer (csp) and restore the previous JSSP, while calls from WASM
    to JS need to compute a new JSSP and restore their CSP after the
    call.

R=ahaas@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1811283003

Cr-Commit-Position: refs/heads/master@{#35096}