[libc++] Always define a key function for std::bad_function_call in the dylib

[libc++] Always define a key function for std::bad_function_call in the dylib

However, whether applications rely on the std::bad_function_call vtable
being in the dylib is still controlled by the ABI macro, since changing
that would be an ABI break.

Differential Revision: https://reviews.llvm.org/D92397