Use an unsigned 64-bit integer for exception class.

This matches the type in other unwind headers (LLVM libunwind,
libcxxrt, glibc).

NB: include/unwind.h is not installed but is only used by libthr

Reviewed by: imp, dim, emaste
Differential Revision: https://reviews.freebsd.org/D34049

(cherry picked from commit b84693501af6521487327ec4eb7c6ae0097ac5d7)