configure: detect --cpu=mipsisa64r6

Treat it as a MIPS64 machine.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

configure

@@ -450,7 +450,7 @@ case "$cpu" in
     linux_arch=loongarch
     ;;
 
-  mips64*)
+  mips64*|mipsisa64*)
     cpu=mips64
     host_arch=mips
     linux_arch=mips