gdbserver: use `gdb::unordered_map`

Replace the few uses of `std::unordered_map` with `gdb::unordered_map`.

The only one of these that is likely to ever see a lot of elements is
probably `process_info::m_ptid_thread_map`.  It was added precisely to
improve performance when there are a lot of threads.  So using
`gdb::unordered_map` here won't hurt.

Change-Id: Ibc4ede5245551fdd7717cb349a012d05726f4363