CONFIG_ARCH_ACORN is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

CONFIG_ARCH_ACORN (not configurable)

configname: CONFIG_ARCH_ACORN

Linux Kernel Configuration
└─>System Type
└─>ARCH_ACORN
In linux kernel since version 2.6.12  

is depended by
CONFIG_ARCH_RPC