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>
diff --git a/target-arm/cpu.c b/target-arm/cpu.c
index f39c19f..a493cc2 100644
--- a/target-arm/cpu.c
+++ b/target-arm/cpu.c
@@ -824,6 +824,8 @@
     cc->do_interrupt = arm_cpu_do_interrupt;
     cc->dump_state = arm_cpu_dump_state;
     cc->set_pc = arm_cpu_set_pc;
+    cc->gdb_read_register = arm_cpu_gdb_read_register;
+    cc->gdb_write_register = arm_cpu_gdb_write_register;
 #ifndef CONFIG_USER_ONLY
     cc->get_phys_page_debug = arm_cpu_get_phys_page_debug;
     cc->vmsd = &vmstate_arm_cpu;