Change unw_word_t to always have the same size as the pointer size

This matches the original libunwind API. This also unifies the
type between ARM EHABI and the other configurations, and allows
getting rid of a number of casts in log messages.

The cursor size updates for ppc and or1k are untested, but
unw_proc_info_t shrinks by 4 uint64_t units on i386 at least.

Differential Revision: https://reviews.llvm.org/D39365

llvm-svn: 316942