Remove ptid_build

This removes ptid_build in favor of simply calling the ptid_t
constructor directly.

gdb/ChangeLog
2018-07-03  Tom Tromey  <tom@tromey.com>

	* common/ptid.h (ptid_build): Don't declare.
	* common/ptid.c (ptid_build): Remove.
	* aix-thread.c: Update.
	* bsd-kvm.c: Update.
	* bsd-uthread.c: Update.
	* common/agent.c: Update.
	* common/ptid.c: Update.
	* common/ptid.h: Update.
	* corelow.c: Update.
	* darwin-nat.c: Update.
	* fbsd-nat.c: Update.
	* gnu-nat.c: Update.
	* linux-fork.c: Update.
	* linux-nat.c: Update.
	* linux-thread-db.c: Update.
	* nat/linux-osdata.c: Update.
	* nat/linux-procfs.c: Update.
	* nto-procfs.c: Update.
	* obsd-nat.c: Update.
	* proc-service.c: Update.
	* procfs.c: Update.
	* ravenscar-thread.c: Update.
	* remote-sim.c: Update.
	* remote.c: Update.
	* sol-thread.c: Update.
	* target.c: Update.
	* windows-nat.c: Update.

gdb/gdbserver/ChangeLog
2018-07-03  Tom Tromey  <tom@tromey.com>

	* linux-low.c: Update.
	* lynx-low.c: Update.
	* nto-low.c: Update.
	* remote-utils.c: Update.
	* spu-low.c: Update.
	* thread-db.c: Update.
	* win32-low.c: Update.

gdb/linux-fork.c

@@ -94,7 +94,7 @@ add_fork (pid_t pid)
     }
 
   fp = XCNEW (struct fork_info);
-  fp->ptid = ptid_build (pid, pid, 0);
+  fp->ptid = ptid_t (pid, pid, 0);
   fp->num = ++highest_fork_num;
 
   if (fork_list == NULL)