CONFIG_GDBSTUB_ON_TTYS0 is not available for thedefaultarchitecture x86.
Result is shown for architecture mn10300

/dev/ttyS0

configname: CONFIG_GDBSTUB_ON_TTYS0

GDB stub port
└─>/dev/ttyS0
In linux kernel since version 2.6.25 (release Date: 2008-04-16)