sw64: kvm: support debugging guest kernel via gdb

Sunway inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I56WV8



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To support qemu-gdb, this patch adds a new exit type EXIT_DEBUG
to handle exits for breakpoint debugging.

Signed-off-by: Lu Feifei <lufeifei@wxiat.com>
Reviewed-by: He Sheng <hesheng@wxiat.com>
Signed-off-by: Gu Zitao <guzitao@wxiat.com>