Real world native libs likely need to access the wasm_runtime_xxx API.
This example demonstrates it.
Build vmlib as a shared lib to make it straightforward to share a
single runtime instance between iwasm and native libs.
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.
