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: NAndreas Färber <afaerber@suse.de>