Do not export start function (#998)

Author: dcodeIO

src/wasm-linker.cpp

@@ -224,7 +224,6 @@ void Linker::layout() {
     auto* func = new Function;
     func->name = start;
     out.wasm.addFunction(func);
-    exportFunction(start, true);
     out.wasm.addStart(start);
     Builder builder(out.wasm);
     auto* block = builder.makeBlock();

test/dot_s/start_main0.wast

@@ -5,7 +5,6 @@
  (export "stackAlloc" (func $stackAlloc))
  (export "stackRestore" (func $stackRestore))
  (export "main" (func $main))
- (export "_start" (func $_start))
  (start $_start)
  (func $main
  )

test/dot_s/start_main2.wast

@@ -5,7 +5,6 @@
  (export "stackAlloc" (func $stackAlloc))
  (export "stackRestore" (func $stackRestore))
  (export "main" (func $main))
- (export "_start" (func $_start))
  (start $_start)
  (func $main (param $0 i32) (param $1 i32) (result i32)
   (return