CONFIG_UML is not available for thedefaultarchitecture x86.
Result is shown for architecture um

CONFIG_UML (not configurable)

configname: CONFIG_UML

Linux Kernel Configuration
└─>UML
In linux kernel since version 2.6.12  
selects
CONFIG_HAVE_GENERIC_HARDIRQS
CONFIG_GENERIC_IRQ_SHOW

is selected by
CONFIG_TR