Use emoji to indicate errors and warnings

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>