[llvm] [cmake] Hardcode some `check_include_file` checks (PR #104706)

[Vlad Serebrennikov via llvm-commits]

Endilll wrote:

> The list of header not found for z/OS is:
> 
>     * <link.h>
> 
>     * <mach/mach.h>
> 
>     * <malloc/malloc.h>
> 
>     * <sys/param.h>
> 
>     * <sysexits.h>

This is exactly the list I came up with after consulting z/OS documentation. Thank you for confirming, and kudos to z/OS documentation writers.

@perry-ca Can you comment on how CMake works on z/OS? In particular, how to guard z/OS-specific code in CMake code. `if (ZOS)`?

https://github.com/llvm/llvm-project/pull/104706