gdb/darwin-nat.c: Fix "cast to pointer from integer of different size" warning

gdb/darwin-nat.c: Fix "cast to pointer from integer of different size" warning

Fixes, with gcc 5.3.0:

 .../src/gdb/darwin-nat.c: In function 'void darwin_resume_thread(inferior*, darwin_thread_t*, int, int)':
 .../src/gdb/darwin-nat.c:731:21: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
     (caddr_t)thread->gdb_port, nsignal);
		      ^
 .../src/gdb/darwin-nat.c:84:35: note: in definition of macro 'PTRACE'
   darwin_ptrace(#CMD, CMD, (PID), (ADDR), (SIG))
				    ^

thread->gdb_port is an unsigned int, caddr_t is a void pointer.

gdb/ChangeLog:
2016-04-20  Pedro Alves  <palves@redhat.com>

	* darwin-nat.c (darwin_resume_thread): Add uintptr_t cast.