Fix sluggish display of symbols in UTF-8 language environment

Fix sluggish display of symbols in UTF-8 language environment

* lisp/international/fontset.el (setup-default-fontset): Make sure
Symbola and FreeMono are set up in the default fontset as belonging
to the "iso10646-1" registry.  In the UTF-8 locale, this avoids a
long and futile search for a suitable font, whose side effect is a
lot of consing, which then hits the font-cache compacting issue,
and causes very sluggish redisplay of characters displayed by
these fonts.  All this happens because the default for the
registry is "iso8859-1".  (Bug#24953)