Avoid crashes at startup on systems that CANNOT_DUMP

Avoid crashes at startup on systems that CANNOT_DUMP

* src/xdisp.c (syms_of_xdisp) <redisplay--inhibit-bidi>: New
boolean variable.
(init_iterator, reseat_to_string)
(Fcurrent_bidi_paragraph_direction)
(Fbidi_find_overridden_directionality): Use
redisplay--inhibit-bidi instead of purify-flag, to determine when
it's safe to reorder bidirectional text.

* lisp/loadup.el (redisplay--inhibit-bidi): Set to t at the
beginning of the file.  Reset to nil when charprop.el is
successfully loaded, or when we are going to dump, whichever
happens last.  (Bug#22975)