Abstract POSIX filesystem functions (#2585)

To allow non-POSIX platforms such as Windows to support WASI libc
filesystem functionality, create a set of wrapper functions which provide a
platform-agnostic interface to interact with the host filesystem. For now,
the Windows implementation is stubbed but this will be implemented
properly in a future PR. There are no functional changes in this change,
just a reorganization of code to move any direct POSIX references out of
posix.c in the libc implementation into posix_file.c under the shared
POSIX sources.

See https://github.com/bytecodealliance/wasm-micro-runtime/issues/2495 for a
more detailed overview of the plan to port the WASI libc filesystem to Windows.

core/iwasm/interpreter/wasm.h

@@ -350,7 +350,7 @@ typedef struct WASIArguments {
     uint32 ns_lookup_count;
     char **argv;
     uint32 argc;
-    int stdio[3];
+    os_raw_file_handle stdio[3];
 } WASIArguments;
 #endif