gdb/testsuite: fix mi-exec-run.exp with native-extended-gdbserver board

gdb/testsuite: fix mi-exec-run.exp with native-extended-gdbserver board

When running with the native-extended-gdbserver board, I currently see
one failure in gdb.mi/mi-exec-run.exp:

    FAIL: gdb.mi/mi-exec-run.exp: inferior-tty=separate: mi=separate: force-fail=0: breakpoint hit reported on console (timeout)

In this test the MI interface should be started in a separate tty,
which means we should have a CLI tty and an MI tty, however, this is
not happening.  Instead GDB is just started in MI mode and there is no
CLI tty.

The test script tries to switch between the CLI an MI terminals and
look for some expected output on each, however, as there is no CLI
terminal the expected output never arrives, and the test times out.

It turns out that this is not a GDB problem, rather, this is an issue
with argument passing within the test script.

The proc default_mi_gdb_start expects to take a set of flags (strings)
as arguments, each of flag is expected to be a separate argument.  The
default_mi_gdb_start proc collects all its arguments into a list using
the special 'args' parameter name, and then iterates over this list to
see which flags were passed.

In mi_gdb_start, which forwards to default_mi_gdb_start, the arguments
are also gathered into the 'args' parameter list, but are then
expanded back to be separate arguments using the eval trick, i.e.:

  proc mi_gdb_start { args } {
    return [eval default_mi_gdb_start $args]
  }

This ensures that when we arrive in default_mi_gdb_start each flag is
a separate argument, rather than appearing as a single list containing
all arguments.

When using the native-extended-gdbserver board however, the file
boards/native-extended-gdbserver.exp is loaded, and this file replaces
the default mi_gdb_start with its own version.

This new mi_gdb_start also gathers the arguments into an 'args' list,
but forgets to expand the arguments out using the eval trick.

As a result, when using the native-extended-gdbserver board, by the
time we get to default_mi_gdb_start, we end up with the args list
containing a single item, which is a list containing all the arguments
the user passed.

What this means is that if the user passes two arguments, then, in
default_mi_gdb_start, instead of seeing two separate arguments, we see
a single argument made by concatenating the two arguments together.

The only place this is a problem is in the test mi-exec-run.exp,
which (as far as I can see) is the only test where we might try to
pass both arguments at the same time.  Currently we think we passed
both arguments to mi_gdb_start, but mi_gdb_start behaves as if no
arguments were passed.

This commit fixes the problem by making use of the eval trick within
the native-extended-gdbserver version of mi_gdb_start.  After this,
the FAIL listed at the top of this message is resolved.