[SystemZ/ZOS] Add header file to encapsulate use of <sysexits.h>

The non-standard header file `<sysexits.h>` provides some return values.
`EX_IOERR` is used to as a special value to signal a broken pipe to the clang driver.
On z/OS Unix System Services, this header file does not exists. This patch

- adds a check for `<sysexits.h>`, removing the dependency on `LLVM_ON_UNIX`
- adds a new header file `llvm/Support/ExitCodes`, which either includes
  `<sysexits.h>` or defines `EX_IOERR`
- updates the users of `EX_IOERR` to include the new header file

Reviewed By: hubert.reinterpretcast

Differential Revision: https://reviews.llvm.org/D83472