CONFIG_GDBSTUB_ON_TTYSM0 is not available for the selected architecture x86.
Result is shown for architecture mn10300

/dev/ttySM0 [SIF0]

configname: CONFIG_GDBSTUB_ON_TTYSM0

GDB stub port
└─>/dev/ttySM0 [SIF0]
In linux kernel since version 2.6.25 (release Date: 2008-04-16)