[libc++] do not define exception handlers when building with supc++

[compnerd]

Summary

terminate_handler and unexpected_handler are defined but not used when
building against libsupc++ as the functions for which they are used are provided
by libsupc++. Simply preprocess them away when building against libsupc++.

src/exception.cpp

   #if defined(LIBCXXRT) || defined(_LIBCPPABI_VERSION)
     #define HAVE_DEPENDENT_EH_ABI 1
   #endif
-#else  // __has_include(<cxxabi.h>)
+#elif !defined(__GLIBCXX__) // __has_include(<cxxabi.h>)
   static std::terminate_handler  __terminate_handler;
   static std::unexpected_handler __unexpected_handler;
 #endif // __has_include(<cxxabi.h>)