Fix the adb_gdb script to honor the --gdb flag for a user specified gdb binary according to the existing usage.

BUG=295807

Review URL: https://codereview.chromium.org/23513083

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226514 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226514