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This allows calling it from other places later. The default
implementation calls `abort()`, but making it weak allows user code to
override this function (either directly, or by including a library like
uclibc++ that implements `std::set_terminate()`).

Note that this does not add a declaration for this function, since the
standard dictates this to be in `<exception>`, but we cannot
meaningfully or completely implement that header, so better leave it to
be overridden by e.g. libraries like uclibc++.