To allow anything to depend on WASI types, including platform-specific
data structures, move the WASI libc filesystem/clock interface into
`platform_api_extension.h`, which leaves just WASI types in
`platform_wasi.h`. And `platform_wasi.h` has been renamed to
`platform_wasi_types.h` to reflect that it only defines types now and no
function declarations. Finally, these changes allow us to remove the
`windows_fdflags` type which was essentially a duplicate of
`__wasi_fdflags_t`.
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.
