gdb: fix darwin-nat build

gdb: fix darwin-nat build

There are two errors of this kind:

      CXX    darwin-nat.o
    /Users/smarchi/src/binutils-gdb/gdb/darwin-nat.c:1175:19: error: format specifies type 'unsigned long' but the argument has type 'ULONGEST' (aka 'unsigned long long') [-Werror,-Wformat]
	 ptid.pid (), ptid.tid ());
		      ^~~~~~~~~~~

Fix them by using ptid_t's to_string method.

Change-Id: I52087d5f7ee0fc01ac8b3f87d4db0217cb0d7cc7