gdb_init argument ARGS is a string rather than a list

The argument ARGS of gdb_init is passed from dejagnu is a string, the
test file name.  In dejagnu/runtest.exp:

proc runtest { test_file_name } {
....
....
        if [info exists tool] {
            if { [info procs "${tool}_init"] != "" } {
                ${tool}_init $test_file_name;
            }
        }
....
}

but inn default_gdb_init (callee of gdb_init), we have

    set gdb_test_file_name [file rootname [file tail [lindex $args 0]]]

In tcl, all actual arguments are combined to a list and assigned to
args.  This code here isn't wrong, but unnecessary, because its caller
(proc runtest) only passes one string to it, and IMO, we don't need
such tricky tcl "args".

I doubt that "[lindex $args 0]" is to be backward compatible with old
dejagnu, but dejagnu-1.4 release started to pass $test_file_name to
${too}_init, as I showed above.  dejagnu-1.4 was released in 2001, and
it should be old enough.  I also tried to check whether gdb testusite
works with dejagnu-1.3 or not, but failed to build dejagnu-1.3 on my
machine.  Supposing GDB testsuite requires at least dejagnu-1.4, this
change should be safe.

This patch is update default_gdb_init to treat ARGS as a string instead
of a list.  Then, 'args' sounds like a list, and this patch also renames
it by 'test_file_name', to align with dejagnu.

gdb/testsuite:

2014-05-20  Yao Qi  <yao@codesourcery.com>

	* lib/gdb.exp (default_gdb_init): Rename argument 'args' by
	'test_file_name'.  Treat args as a string instead of a list.
	(gdb_init): Rename argument 'args' by 'test_file_name'.
Set timeout for gdb.reverse/*.exp test cases

Hi,
This patch is to add a new board setting gdb_reverse_timeout, which is
used to set timeout for all gdb.reverse test cases, which are usually
very slow and cause some TIMEOUT failures, for example, on some arm
boards.  We have some alternatives to this approach, but I am not
satisfied with them:

 - Increase the timeout value.  This is the global change, and it may
   cause some delay where actual failures happen.
 - Set timeout by gdb_reverse_timeout in every gdb.reverse/*.exp.
   Then, we have to touch every file under gdb.reverse.

In this patch, we choose a central place to set timeout for all tests
in gdb.reverse, which is convenient.

gdb/testsuite:

2014-05-20  Yao Qi  <yao@codesourcery.com>

	* lib/gdb.exp (gdb_init): Set timeout if test file is under
	gdb.reverse directory and gdb_reverse_timeout exists in board
	setting.
	* README: Document gdb_reverse_timeout.