[z/OS] Improve compiler options on z/OS (llvm#166415)

`_XPLATFORM_SOURCE` needs to be defined to improve source code
compatibility (e.g. for `O_CLOEXEC`). The define `_UNIX03_THREADS` can
be removed, because it is automatically set by `_XOPEN_SOURCE=600`.

See the documentation of feature test macros:
https://www.ibm.com/docs/en/zos/3.1.0?topic=files-feature-test-macros

Tested on z/OS 3.1 with the Open XL C/C++ 2.2 compiler.

Author: redstar

llvm/CMakeLists.txt

@@ -1264,10 +1264,10 @@ endif()
 # Build with _XOPEN_SOURCE on z/OS.
 if (CMAKE_SYSTEM_NAME MATCHES "OS390")
   add_compile_definitions(_XOPEN_SOURCE=600)
+  add_compile_definitions(_XPLATFORM_SOURCE) # Needed e.g. for O_CLOEXEC.
   add_compile_definitions(_OPEN_SYS) # Needed for process information.
   add_compile_definitions(_OPEN_SYS_FILE_EXT) # Needed for EBCDIC I/O.
   add_compile_definitions(_EXT) # Needed for file data.
-  add_compile_definitions(_UNIX03_THREADS) # Multithreading support.
   # Need to build LLVM as ASCII application.
   # This can't be a global setting because other projects may
   # need to be built in EBCDIC mode.