gdbserver: small cleanup of README file

Fix a few outdated or incoherent things in the README:

- Don't mention remote.c nor *-stub.c files as references for the remote
  protocol.  remote.c is in GDB, not GDBserver, and *-stub.c files don't
  exist today.  Add a link to the documentation instead.

- In the "server (target) side" section, use `:2345` instead of
  `host:2345`.  It currently says that using `host:2345` means we would
  expect a connection from `host`.  That's not what I would expect by
  passing a host part here.  If I passed `11.22.33.44:2345` as the listen
  address, I would expect it to instruct gdbserver to listen only on that
  (11.22.33.44) network interface, not to expect a connection from host
  `11.22.33.44`.  So, remove that part of the sentence.

- Remove the list of supported target, refer to configure.srv instead.
  Keeping a list here is bound to lose sync with reality.

- In the cross-compile instructions, I don't think it's necessary to mention
  "In a Bourne shell".

- In the cross-compile instructions, I don't know what passing
  `your-target-name` to configure does, I don't think it's valid.  Use
  `make all-gdbserver` as in the instructions just above.

gdbserver/ChangeLog:

	* README: Fix a few outdated or incoherent things.

Change-Id: I79349e25bc1bc53447855e0dea6cc7b9630f4553