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.