Skip round-trip tests for structs with FAMs

[bossmc]

For example:

strcut inotify_event {
  int      wd;       /* Watch descriptor */
  uint32_t mask;     /* Mask describing event */
  uint32_t cookie;   /* Unique cookie associating related
                        events (for rename(2)) */
  uint32_t len;      /* Size of name field */
  char     name[];   /* Optional null-terminated name */
};

the name field at the end of this struct is a Flexible Array Member (FAM - https://en.wikipedia.org/wiki/Flexible_array_member) and represents an inline array of some length (in this case the len field gives the length, but FAMs in general are unsized). These forms cause problems in two ways:

1. There's no rust-equivalent for FAM-containing structs (see Extern types v2 rfcs#3396 for details) - the current approach is just to omit the name field in the Rust representation.
2. These structs cannot be passed by value (basically the compiler cannot know how many elements of the name field should be copied) and different compilers do different things when asked to do so.

This PR focusses on the second of these problems since it was causing test failures on my machine. If you run the libc-test suite with GCC as your C compiler you get a compiler note saying the following:

/out/main.c: In function ‘__test_roundtrip_inotify_event’:
/out/main.c:21011:13: note: the ABI of passing struct with a flexible array member has changed in GCC 4.4

and the test suite passes. OTOH if you build with clang as your compiler you get no compile time warnings/errors but the test suite fails:

size of struct inotify_event is 16 in C and 185207048 in Rust
error: test failed, to rerun pass `--test main`

Caused by:
  process didn't exit successfully: `/deps/main-e32ea4d2acb868af` (signal: 11, SIGSEGV: invalid memory reference)

(note that 185207048 is 0x08090A0B (which is what the round-tripped value is initialized with by the test suite) so clearly the Rust and C code are disagreeing about the calling convention/register layout when passing such a struct).

Given that there doesn't seem to be a specification for passing these objects by value, this PR simply omits them from the roundtrip tests and the test suite now passes on GCC and clang without warnings.

[rustbot]

r? @JohnTitor

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[JohnTitor]

Thanks! @bors r+

[bors]

📌 Commit f8e82dc has been approved by JohnTitor

It is now in the queue for this repository.

[bors]

⌛ Testing commit f8e82dc with merge e2d90cd...

[bors]

☀️ Test successful - checks-actions, checks-cirrus-freebsd-12, checks-cirrus-freebsd-13, checks-cirrus-freebsd-14
Approved by: JohnTitor
Pushing e2d90cd to main...