[libc++] Clean up mess around __throw_runtime_error

We were defining the function in locale.cpp, and we actually had two
overloads for it. This is pretty confusing given that one was static
and not exported from the dylib, and the other one was. Instead, use
the vanilla __throw_runtime_error function everywhere even though that
adds a tiny bit of code duplication.

Differential Revision: https://reviews.llvm.org/D155008