hw/core/cpu: gdb_arch_name string should not be freed

hw/core/cpu: gdb_arch_name string should not be freed

The documentation for the CPUClass::gdb_arch_name method claims that
the returned string should be freed with g_free().  This is not
correct: in commit a650683871ba728 we changed this method to
instead return a simple constant string, but forgot to update
the documentation.

Make the documentation match the new semantics.

Fixes: a650683871ba728 ("hw/core/cpu: Return static value with gdb_arch_name()")
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20250317142819.900029-2-peter.maydell@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>