cpu: Introduce CPUClass::gdb_{read,write}_register()

Completes migration of target-specific code to new target-*/gdbstub.c.

Acked-by: Michael Walle <michael@walle.cc> (for lm32)
Acked-by: Max Filippov <jcmvbkbc@gmail.com> (for xtensa)
Signed-off-by: Andreas Färber <afaerber@suse.de>

include/exec/gdbstub.h

@@ -84,6 +84,14 @@ int gdbserver_start(int);
 int gdbserver_start(const char *port);
 #endif
 
+/**
+ * gdb_has_xml:
+ * This is an ugly hack to cope with both new and old gdb.
+ * If gdb sends qXfer:features:read then assume we're talking to a newish
+ * gdb that understands target descriptions.
+ */
+extern bool gdb_has_xml;
+
 /* in gdbstub-xml.c, generated by scripts/feature_to_c.sh */
 extern const char *const xml_builtin[][2];