[SystemZ][z/OS] Build in ASCII 64 bit mode on z/OS (#90630)

Setting the correct build flags on z/OS to build LLVM as 64-bit ASCII
application.

Author: fanbo-meng

llvm/CMakeLists.txt

@@ -1158,6 +1158,11 @@ if (CMAKE_SYSTEM_NAME MATCHES "OS390")
   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.
+  append("-fzos-le-char-mode=ascii" CMAKE_CXX_FLAGS CMAKE_C_FLAGS)
+  append("-m64" CMAKE_CXX_FLAGS CMAKE_C_FLAGS)
 endif()
 
 # Build with _FILE_OFFSET_BITS=64 on Solaris to match g++ >= 9.