gdbstub: Allow gdb_core_xml_file to be set at runtime

gdbstub: Allow gdb_core_xml_file to be set at runtime

Currently the CPUClass:gdb_core_xml_file setting is a simple 'const
char *' which the CPU class must set to a fixed string.  Allow the
CPU class to instead set a new method gdb_get_core_xml_file() which
returns this string.

This will allow Arm CPUs to use different XML files for AArch32 vs
AArch64 without having to have an extra AArch64-specific class type
purely to give somewhere to set cc->gdb_core_xml_file differently.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20250317142819.900029-3-peter.maydell@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>