Replace deprecated_target_wait_hook by observers

Replace deprecated_target_wait_hook by observers

Commit b60cea7 (Make target_wait options use enum flags) broke
deprecated_target_wait_hook usage: there's a commit comment telling
this hook has not been converted.

Rather than trying to mend it, this patch replaces the hook by two
target_wait observers:

target_pre_wait (ptid_t ptid)
target_post_wait (ptid_t event_ptid)

Upon target_wait entry, target_pre_wait is notified with the ptid
passed to target_wait. Upon exit, target_post_wait is notified with
the event ptid returned by target_wait. Should an exception occur,
event_ptid is null_ptid.

This change benefits to Insight (out-of-tree): there's no real use of the
late hook in gdb itself.