[libc++] NFC: only fwd-declare ABI-functions in exception_ptr.h condi…

…tionally

This patch fixes the unconditional fodward-declarations of ABI-functions in
exception_ptr.h, and makes it dependendent on the availability macro, as it
should've been from the beginning.
The declarations being unconditional break the build with libcxxrt
before 045c52ce821388f4ae4d119fe4fb75f1eb547b85, now the are opt-out.

* libcxx/include/__exception/exception_ptr.h
  Only forward-declare ABI-functions if they are meant to be used.

libcxx/include/__exception/exception_ptr.h

@@ -26,6 +26,8 @@
 
 #ifndef _LIBCPP_ABI_MICROSOFT
 
+#  if _LIBCPP_AVAILABILITY_HAS_INIT_PRIMARY_EXCEPTION
+
 namespace __cxxabiv1 {
 
 extern "C" {
@@ -37,14 +39,16 @@ _LIBCPP_OVERRIDABLE_FUNC_VIS __cxa_exception* __cxa_init_primary_exception(
     void*,
     std::type_info*,
     void(
-#  if defined(_WIN32)
+#    if defined(_WIN32)
         __thiscall
-#  endif
+#    endif
             *)(void*)) throw();
 }
 
 } // namespace __cxxabiv1
 
+#  endif
+
 #endif
 
 namespace std { // purposefully not using versioning namespace