This changes the mechanism for verbose termination (again!) to make it
support compile-time customization in addition to link-time customization,
which is important for users who need fine-grained control over what code
gets generated around sites that call the verbose termination handler.

This concern had been raised to me both privately by prospecting users
and in https://llvm.org/D140944, so I think it is clearly worth fixing.

We still support _LIBCPP_AVAILABILITY_CUSTOM_VERBOSE_ABORT_PROVIDED for
a limited time since the same functionality can be achieved by overriding
the _LIBCPP_VERBOSE_ABORT macro.

Differential Revision: https://reviews.llvm.org/D141326