When WAMR_BUILD_TARGET isn't set, choosing right target is decided
by checking `CMAKE_SIZEOF_VOID_P` variable. However, choosing `X86_32`
target is not doing specifically checking size of void pointer. It is kind
a fallback target for others.
This patch explicitly checks the size of void pointer before setting the target
to `X86_32` to fix the issue.
Refer to [Networking API design](https://github.com/WebAssembly/WASI/issues/370)
and [feat(socket): berkeley socket API v2](https://github.com/WebAssembly/WASI/pull/459):
- Support the socket API of synchronous mode, including `socket/bind/listen/accept/send/recv/close/shutdown`,
the asynchronous mode isn't supported yet.
- Support adding `--addr-pool=<pool1,pool2,..>` argument for command line to identify the valid ip address range
- Add socket-api sample and update the document
