[libcxxabi] define _LIBCPP_VERBOSE_ABORT

libc++ may be built with or without assertions. This causes definitions
of various methods of std::string_view to contain assertions and calls
to __libcpp_verbose_abort which libcxxabi does not provide. libcxxabi
does provide abort_message with the same interface, so define
_LIBCPP_VERBOSE_ABORT to use that. Otherwise D148566 will trigger
linkage failures for the missing symbol __libcpp_verbose_abort.

Link: https://libcxx.llvm.org/UsingLibcxx.html#enabling-the-safe-libc-mode

Reviewed By: #libc_abi, philnik, ldionne

Differential Revision: https://reviews.llvm.org/D149092