[SystemZ][z/OS] Fix macro (#74878)

This fixes the macro syntax

Author: abhina-sree

llvm/CMakeLists.txt

@@ -1130,7 +1130,7 @@ if (CMAKE_SYSTEM_NAME MATCHES "OS390")
   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("-D_UNIX03_THREADS") # Multithreading support.
+  add_compile_definitions(_UNIX03_THREADS) # Multithreading support.
 endif()
 
 # Build with _FILE_OFFSET_BITS=64 on Solaris to match g++ >= 9.