Use emoji to indicate errors and warnings

This patch adds, at long last, some emoji output to gdb.  In
particular, warnings are indicated with the U+26A0 (WARNING SIGN), and
errors with U+274C (CROSS MARK).

There is a new setting to control whether emoji output can be used.
It defaults to "auto", which means emoji will be used if the host
charset is UTF-8.  Note that disabling styling will also disable
emoji, handy for traditionalists.

I've refactored mingw console output a little, so that emoji will not
be printed to the console.  Note the previous code here was a bit
strange in that it assumed that the first use of gdb_console_fputs
would be to stdout.

This version lets the user control the prefixes directly, so different
emoji can be chosen if desired.

Reviewed-By: Eli Zaretskii <eliz@gnu.org>
Reviewed-By: Keith Seitz <keiths@redhat.com>
Reviewed-By: Guinevere Larsen <guinevere@redhat.com>

gdb/exceptions.c

@@ -25,6 +25,7 @@
 #include "serial.h"
 #include "ui.h"
 #include <optional>
+#include "cli/cli-style.h"
 
 static void
 print_flush (void)
@@ -105,6 +106,7 @@ exception_print (struct ui_file *file, const struct gdb_exception &e)
   if (e.reason < 0 && e.message != NULL)
     {
       print_flush ();
+      print_error_prefix (file);
       print_exception (file, e);
     }
 }
@@ -118,6 +120,7 @@ exception_fprintf (struct ui_file *file, const struct gdb_exception &e,
       va_list args;
 
       print_flush ();
+      print_error_prefix (file);
 
       /* Print the prefix.  */
       va_start (args, prefix);