this was problematic because several archs don't define __WORDSIZE. we
could add it, but I would rather phase this macro out in the long
term. in our version of the headers, UINTPTR_MAX is available here, so
just use it instead.
these structures are purely for use by trace/debug tools and tools
working with core files. the definition of fpregset_t, which was
previously here, has been removed because it was wrong; fpregset_t
should be the type used in mcontext_t, not the type used in
ptrace/core stuff.
