[lld][WebAssembly] Limit size of shared 64-bit memories of 2^^34

[sbc100]

Summary

This is current limit in v8. See
https://github.com/WebAssembly/memory64/issues/33 how we might change
this in the future.

[sbc100]

Working on adding a test now. This is needed to run wasm64 + pthread + dynamic linking tests on the emscripten side.

[dschuff]

LGTM with a test.

lld/wasm/Writer.cpp

if (WasmSym::heapBase) {
     // grow the heap at runtime.
     // We'll align the heap base here because memory allocators might expect
     // __heap_base to be aligned already.
     memoryPtr = alignTo(memoryPtr, heapAlignment);
     log("mem: heap base   = " + Twine(memoryPtr));
     WasmSym::heapBase->setVA(memoryPtr);
   }
 
-  uint64_t maxMemorySetting = 1ULL << (config->is64.value_or(false) ? 48 : 32);
+  uint64_t maxMemorySetting = 1ULL << 32;
+  if (config->is64.value_or(false)) {
+    // TODO: Update once we decide on a reasonable limit here:
+    // https://github.com/WebAssembly/memory64/issues/33
+    maxMemorySetting = 1ULL << 34;
+  }
 
   if (config->initialMemory != 0) {
     if (config->initialMemory != alignTo(config->initialMemory, WasmPageSize))
       error("initial memory must be " + Twine(WasmPageSize) + "-byte aligned");
     if (memoryPtr > config->initialMemory)
       error("initial memory too small, " + Twine(memoryPtr) + " bytes needed");
     if (config->initialMemory > maxMemorySetting)
       error("initial memory too large, cannot be greater than " +