Fix couple of test failures when using the LIBCXXABI_SILENT_TERMINATE mode.

When libcxxabi is built in LIBCXXABI_SILENT_TERMINATE mode, libcxx test suite reports
two failures:

  std/depr/exception.unexpected/set.unexpected/get_unexpected.pass.cpp
  std/depr/exception.unexpected/set.unexpected/set_unexpected.pass.cpp

This is because the default unexpected handler is set to std::abort instead of
std::terminate which these tests expect.

llvm-svn: 295175