Since these are defined in C as macros, they must be reimplemented in
libc as Rust functions. They're hard to get exactly right, and they
vary from platform to platform. The test builds custom C code that uses
the real macros, and compares its output to the Rust versions' output
for various inputs.
Skip the CMSG_NXTHDR test on sparc64 linux because it hits a Bus Error.
Issue #1239
Skip the entire cmsg test program on s390x because it dumps core
seemingly before the kernel finishes booting.
Issue #1240
