ctest: Support flexible array members

[tgross35]

C's "flexible array members" are basically a convenient way to access a properly-aligned pointer to data after the struct:

struct s {
    int len;
    int data[];
};

In Rust we typically do:

struct S {
    len: c_int,
    data: [c_int; 0],
}

We have a handful of tests that are related to FAM not being supported, e.g.

libc/libc-test/build.rs

Lines 5338 to 5345 in 93a17ee

	// The following types contain Flexible Array Member fields which have unspecified calling
	// convention. The roundtripping tests deliberately pass the structs by value to check "by
	// value" layout consistency, but this would be UB for the these types.
	"inotify_event" => true,
	"fanotify_event_info_fid" => true,
	"cmsghdr" => true,
	"bcm_msg_head" => true,

. I think this should be pretty easy to start supporting.

Cc of course @mbyx