gdbsupport: add type definitions for pid, lwp and tid

A following patch will want to declare variables of the same type as
some ptid_t components.  To make that easy (and avoid harcoding those
types everywhere), define some type definitions in the ptid_t struct for
each of them.  Use them throughout ptid.h.

I initially used pid_t, lwp_t and tid_t, but there is the risk of some
system defining the pid_t type using a macro instead of a typedef, which
would break things.  So, use the _type suffix instead.

Change-Id: I820b0bea9dafcb4914f1c9ba4bb96b5c666c8dec
Approved-By: Andrew Burgess <aburgess@redhat.com>