Introduce target_is_gdbserver

This patch introduces a function in gdbserver-support.exp to find out
whether the current target is GDBserver.

The code was inspired from gdb.trace/qtro.exp, so it replaces the code
there by a call to the new function.

New in v5:

	* Move target_is_gdbserver to gdb.exp.
	* Fix formatting and typos.

gdb/testsuite/ChangeLog:

	* gdb.trace/qtro.exp: Replace gdbserver detection code by...
	* lib/gdb.exp (target_is_gdbserver): New
	procedure.