ctest: support flexible array members

[Deepak-negi11]

Fixes #5369

ctest couldn't test fields like uint8_t data[] (we write them as [T; 0]), so libc-test was skipping them.

What I changed:

• ctest skips only the size check for [T; 0] fields, since C can't do sizeof on them. offset and type are still checked
• [T; 0] is now written as T[] in the generated C, because MSVC errors on [0] arrays
• added a VarBuf struct to the ctest tests. I skipped its roundtrip test because it segfaulted on FreeBSD when passed by value
• removed the FAM skips in libc-test (dirent.d_name on wasi, iw_encode_ext.key, fanotify_event_info_fid.handle, mcontext_t.__extcontext on musl loongarch64, bcm_msg_head.frames, af_alg_iv.iv, file_handle.f_handle)

I didn't touch roundtrip for the libc structs. not sure if ctest should skip those automatically, let me know and I can do it separately.

I ran the full CI on my fork and the tests passed: https://github.com/Deepak-negi11/libc/actions/runs/38123417531 (only Verify build failed there, because it needs main as the base branch)

• Relevant tests in ctest/tests and ctest/src/tests.rs have been updated
• Tested locally (LIBC_BLESS=1 cargo test -p ctest)

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