Revert "gdb: remove unnecessary parameter wait_ptid from do_target_wait"

This reverts commit ac0d67ed.

There was nothing wrong with the commit which I'm reverting here, but
it removed some functionality that will be needed for a later commit;
that is, the ability for GDB to ask for events from a specific ptid_t
via the do_target_wait function.

In a follow up commit, this functionality will be used to implement
inferior function calls in multi-threaded inferiors.