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/common/posix/platform_api_posix.cmake

@@ -5,4 +5,11 @@ set (PLATFORM_COMMON_POSIX_DIR ${CMAKE_CURRENT_LIST_DIR})
 
 file (GLOB_RECURSE source_all ${PLATFORM_COMMON_POSIX_DIR}/*.c)
 
+if (NOT WAMR_BUILD_LIBC_WASI EQUAL 1)
+    list(REMOVE_ITEM source_all ${PLATFORM_COMMON_POSIX_DIR}/posix_file.c)
+else()
+    include (${CMAKE_CURRENT_LIST_DIR}/../libc-util/platform_common_libc_util.cmake)
+    set(source_all ${source_all} ${PLATFORM_COMMON_LIBC_UTIL_SOURCE})
+endif()
+
 set (PLATFORM_COMMON_POSIX_SOURCE ${source_all} )