disable _LIBCPP_AVAILABILITY_HAS_INIT_PRIMARY_EXCEPTION unconditionally

When _LIBCPP_HAS_NO_VENDOR_AVAILABILITY_ANNOTATIONS is not defined, things
work as expected: i.e. the feature is only used on macOS that vendors LLVM>=18.

Our general policy of setting `-D_LIBCPP_DISABLE_AVAILABILITY` (which causes
the above macro to be defined) wherever newer C++ features are needed works
fine because we ship our own libcxx. In this case however, the required symbols
(`___cxa_init_primary_exception` etc.) is in libcxxabi, which we intentionally
don't ship ourselves.

Therefore, we must disable this feature unconditionally, at least until our
macOS baseline is >=15.0, as we rely on the libcxxabi shipped by the system.

See also llvm#77653 & llvm#86077

Author: h-vetinari

libcxx/include/__configuration/availability.h

@@ -352,8 +352,8 @@
 // These macros controls the availability of __cxa_init_primary_exception
 // in the built library, which std::make_exception_ptr might use
 // (see libcxx/include/__exception/exception_ptr.h).
-#define _LIBCPP_AVAILABILITY_HAS_INIT_PRIMARY_EXCEPTION _LIBCPP_INTRODUCED_IN_LLVM_18
-#define _LIBCPP_AVAILABILITY_INIT_PRIMARY_EXCEPTION _LIBCPP_INTRODUCED_IN_LLVM_18_ATTRIBUTE
+#define _LIBCPP_AVAILABILITY_HAS_INIT_PRIMARY_EXCEPTION 0
+#define _LIBCPP_AVAILABILITY_INIT_PRIMARY_EXCEPTION /* nothing */
 
 // This controls the availability of C++23 <print>, which
 // has a dependency on the built library (it needs access to