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.

This is not a straight revert of the above commit.  Reverting the
above commit replaces a 'nullptr' with 'NULL', I've gone in and
changed that, preserving the 'nullptr'.

Reviewed-By: Tankut Baris Aktemur <tankut.baris.aktemur@intel.com>
Tested-By: Luis Machado <luis.machado@arm.com>
Tested-By: Keith Seitz <keiths@redhat.com>