[tilegx] Improve previous structure type return value handling.

  This fix assume the returnd structure type is always < 80bytes, as
it's type of class Handle<XXX>.

  FIXME:

  It's better we get the exact return type and do check, but looks like
there is no way to get the return type info currently.

  Mark as FIXME for later cleanup.
[tilegx] fix is_intn bug.

  TileGX is 64bit processor, while v8's default is_intXXX macro only
support 32bit processor. we should use is_lintn instead of is_intn,
otherwise some operand are truncated unexpectedly.