wasm: bump dyld v8 heap size limit

This patch adds `--max-old-space-size=8192` to wasm dyld shebang
arguments to bump V8 heap size limit. The default limit
(`heap_size_limit` returned by `v8.getHeapStatistics()`) is
dynamically determined and a bit too low under certain workloads, and
V8 would waste too much CPU time to garbage collect old generation
heap more aggressively. Bumping the limit to 8G doesn't imply dyld
would really take that much memory at run-time, but it lessens V8 heap
stress significantly.