Readd the null check

Author: petrhosek

libc/cmake/modules/LLVMLibCTestRules.cmake

@@ -3,6 +3,11 @@ function(_get_common_test_compile_options output_var c_test flags)
   _get_compile_options_from_config(config_flags)
   _get_compile_options_from_arch(arch_flags)
 
+  # Death test executor is only available in Linux for now.
+  if(NOT ${LIBC_TARGET_OS} STREQUAL "linux")
+    list(REMOVE_ITEM config_flags "-DLIBC_ADD_NULL_CHECKS")
+  endif()
+
   set(compile_options
       ${LIBC_COMPILE_OPTIONS_DEFAULT}
       ${LIBC_TEST_COMPILE_OPTIONS_DEFAULT}