See https://bugs.llvm.org/show_bug.cgi?id=35945, which reports that libc++ doesn't build for the Raspberry PI.
The problem is that the exception class is defined as a char[8] in that library, rather than uint64_t, as in other libraries.
Same size, same values, different type.
Solution: Pull the code to check the exception class into a routine, and memcpy the field into a uint64_t and call the routine everywhere we were previously checking directly.