Small changes to libpthread to better enable running on !NetBSD

These are, apart from maybe the bit about not including
pthread_cancelstub.c, more or less ready to go into NetBSD.

1) allow not including pthread_cancelstub.c.  It wreaks havoc with how
   rumprun renames symbols
2) if _PLATFORM_GETTCB is defined, call that instead of relying of
   suitably set up registers
3) to avoid having to translate between NetBSD ucontext_t and
   whatever the host platform uses, allow compiletime override of
   the call to _lwp_makecontext()