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)

lisp/international/fontset.el

@@ -812,10 +812,11 @@
              (#x1F700 . #x1F77F)	;; Alchemical Symbols
              (#x1F780 . #x1F7FF)	;; Geometric Shapes Extended
              (#x1F800 . #x1F8FF)))	;; Supplemental Arrows-C
-    (set-fontset-font "fontset-default" symbol-subgroup "Symbola" nil 'prepend))
+    (set-fontset-font "fontset-default" symbol-subgroup
+                      '("Symbola" . "iso10646-1") nil 'prepend))
   ;; Box Drawing and Block Elements
   (set-fontset-font "fontset-default" '(#x2500 . #x259F)
-                    "FreeMono" nil 'prepend)
+                    '("FreeMono" . "iso10646-1") nil 'prepend)
 
   ;; Since standard-fontset-spec on X uses fixed-medium font, which
   ;; gets mapped to a iso8859-1 variant, we would like to prefer its