Convert default_child_has_foo functions to process_stratum_target methods
This patch converts the default_child_has_foo functions to
process_stratum_target methods. This simplifies "regular"
non-inf_child process_stratum targets, since they no longer have to
override the target_ops::has_foo methods to call the default_child_foo
functions. A couple targets need to override the new defaults
(corelow and tracefiles), but it still seems like a good tradeoff,
since those are expected to be little different (target doesn't run).
gdb/ChangeLog:
2018-11-30 Pedro Alves <palves@redhat.com>