microblaze-tdep: Add ATTRIBUTE_PRINTF to microblaze_debug

microblaze-tdep: Add ATTRIBUTE_PRINTF to microblaze_debug

I am getting this warning with clang:

/home/emaisin/src/binutils-gdb/gdb/microblaze-tdep.c:94:28: error: format string is not a string literal [-Werror,-Wformat-nonliteral]
       vprintf_unfiltered (fmt, args);
                           ^~~

Adding ATTRIBUTE_PRINTF to microblaze_debug gets rid of it.  Strangely,
gcc doesn't warn about non-literal format strings when calling vprintf
(or a vprintf-style function, like vprintf_unfiltered).  I filed this
gcc bug:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82206

gdb/ChangeLog:

	* microblaze-tdep.c (microblaze_debug): Add ATTRIBUTE_PRINTF.