runtime: manually initialize xorshift state

This ensures:

 1. The xorshift state is initialized during interp.
 2. The xorshift state gets initialized to a real random number on
    hardware that supports it at runtime.

This fixes a big binary size regression from the previous commit. It's
still not perfect: most programs increase binary size by a few bytes.
But it's not nearly as bad as before.

src/runtime/runtime_wasmentry.go

@@ -35,6 +35,7 @@ func wasmEntryReactor() {
 	heapStart = uintptr(unsafe.Pointer(&heapStartSymbol))
 	heapEnd = uintptr(wasm_memory_size(0) * wasmPageSize)
 	initHeap()
+	initRand()
 
 	if hasScheduler {
 		// A package initializer might do funky stuff like start a goroutine and