libstdc++: use updated type for __unexpected_handler

Commit f4130a3e changed the type of
__expected_handler in libsupc++/unwind-cxx.h to be a
std::terminate_handler to avoid a deprecated warning. However, the
definition in eh_unex_handler.cc still used the old type
(std::unexpected_handler) and thus causes a warning when compiling
libstdc++ with -Wdeprecated-declarations (which is the default, for
example, for clang).

Adapt the definition to match the declaration.

libstdc++-v3/ChangeLog:

	* libsupc++/eh_unex_handler.cc: Adjust definition type to
	declaration.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>