gdb/testsuite: make gdb.tui/gdb.sh work with dash

gdb/testsuite: make gdb.tui/gdb.sh work with dash

gdb.tui/gdb.sh (used by test gdb.tui/tuiterm-2.exp) doesn't run well
when /bin/sh is dash:

    $ bash testsuite/gdb.tui/gdb.sh
    foo^C
    $ dash testsuite/gdb.tui/gdb.sh
    footestsuite/gdb.tui/gdb.sh: 20: read: arg count

shellcheck actually points it out:

    In testsuite/gdb.tui/gdb.sh line 20:
    read
    ^--^ SC3061 (warning): In POSIX sh, read without a variable is undefined.
    ^--^ SC2162 (info): read without -r will mangle backslashes.

Fix the issue by passing the `_` dummy variable.  Using that particular
variable will not cause shellcheck to whine about it being unused.  Also
add `-r` to make it happy.

Change-Id: Ida3eddbfa3473487743fc96615baf735b4773738