remove $^U dependent behaviors in runtime; chr() and sprintf('%c',...)

remove $^U dependent behaviors in runtime; chr() and sprintf('%c',...)
now return bytes all the way to 255, they will be transparently
coerced (in future) to UTF-8 when they are used in operations
involving other UTF-8 strings; C<use utf8> doesn't set $^U anymore

p4raw-id: //depot/perl@5013