CONFIG_XMON is not available for thedefaultarchitecture x86.
Result is shown for architecture powerpc

Include xmon kernel debugger

configname: CONFIG_XMON

Linux Kernel Configuration
└─>Include xmon kernel debugger
In linux kernel since version 2.6.12  
Include in-kernel hooks for the xmon kernel monitor/debugger.
Unless you are intending to debug the kernel, say N here.
Make sure to enable also CONFIG_BOOTX_TEXT on Macs. Otherwise
nothing will appear on the screen (xmon writes directly to the
framebuffer memory).
The cmdline option 'xmon' or 'xmon=early' will drop into xmon
very early during boot. 'xmon=on' will just enable the xmon
debugger hooks. 'xmon=off' will disable the debugger hooks
xmon will print a backtrace on the very first invocation.
'xmon=nobt' will disable this autobacktrace.