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/shared/platform/riot/platform_internal.h

@@ -52,6 +52,10 @@ typedef struct korp_cond {
 #define os_printf printf
 #define os_vprintf vprintf
 
+typedef int os_file_handle;
+typedef DIR *os_dir_stream;
+typedef int os_raw_file_handle;
+
 #if WA_MATH
 /* clang-format off */
 /* math functions which are not provided by os*/