Fix failure in mjsunit/wasm/embenchen/box2d on 32-bit architectures that do not support unaligned access.

This test fails because WasmGraphBuilder::BuildCFuncInstruction allocates
space for doubles using StackSlot turbofan operator, but this space is not
guaranteed to be 8 bytes aligned if SP itself is not 8 bytes aligned (which
is the case on 32-bit architectures).

BUG=mjsunit/wasm/embenchen/box2d

Review-Url: https://codereview.chromium.org/2177863002
Cr-Commit-Position: refs/heads/master@{#38039}