Fix struct stat fields for darwin

[kateinoigakukun]

I'm creating a wamr binding library for Swift and I faced the following errors

$ swift build --target wamr-demo
./wamr-swift/Sources/wamr-core-darwin/wamr/core/iwasm/libraries/libc-wasi/sandboxed-system-primitives/src/posix.c:2100:40: error: no member named 'st_atsim' in 'struct stat'
      .st_atim = convert_timespec(&in->st_atsim),
                                   ~~  ^
./wamr-swift/Sources/wamr-core-darwin/wamr/core/iwasm/libraries/libc-wasi/: error: no member named 'st_mtim' in 'struct stat'
      .st_mtim = convert_timespec(&in->st_mtim),
                                   ~~  ^
./wamr-swift/Sources/wamr-core-darwin/wamr/core/iwasm/libraries/libc-wasi/sandboxed-system-primitives/src/posix.c:2102:40: error: no member named 'st_ctim' in 'struct stat'
      .st_ctim = convert_timespec(&in->st_ctim),

These no member errors are only reproducible when building with -fmodule because darwin doesn't support POSIX compatible stat fields when -fmodule. So I added macros for platform compatibility.

[kateinoigakukun]

Thanks for quick merge 😄