[libc++abi] Mark __cxa_new_handler with _LIBCPP_SAFE_STATIC

For consistency with the other handlers, and because requiring constant
initialization whenever we can is a good thing.

Differential Revision: https://reviews.llvm.org/D110866

libcxxabi/src/cxa_default_handlers.cpp

@@ -105,7 +105,8 @@ _LIBCPP_SAFE_STATIC std::terminate_handler __cxa_terminate_handler = default_ter
 _LIBCXXABI_DATA_VIS
 _LIBCPP_SAFE_STATIC std::unexpected_handler __cxa_unexpected_handler = default_unexpected_handler;
 
-std::new_handler __cxa_new_handler = 0;
+_LIBCXXABI_DATA_VIS
+_LIBCPP_SAFE_STATIC std::new_handler __cxa_new_handler = 0;
 
 namespace std
 {