WASI & darwin: support basic file io based on libc

Signed-off-by: Takeshi Yoneda <takeshi@tetrate.io>

src/runtime/runtime_wasm_wasi.go

@@ -8,11 +8,16 @@ import (
 
 type timeUnit int64
 
+// libc constructors
+//export __wasm_call_ctors
+func __wasm_call_ctors()
+
 //export _start
 func _start() {
 	// These need to be initialized early so that the heap can be initialized.
 	heapStart = uintptr(unsafe.Pointer(&heapStartSymbol))
 	heapEnd = uintptr(wasm_memory_size(0) * wasmPageSize)
+	__wasm_call_ctors()
 	run()
 }