CONFIG_ARC_TUNE_MCPU is not available for thedefaultarchitecture x86.
Result is shown for architecture arc

Override default -mcpu compiler flag


Linux Kernel Configuration
└─>ARC Architecture Configuration
└─>ARC CPU Configuration
└─>Override default -mcpu compiler flag
In linux kernel since version 5.1 (release Date: 2019-05-05)  
Override default -mcpu=xxx compiler flag (which is set depending on
the ISA version) with the specified value.
NOTE: If specified flag isn't supported by current compiler the
ISA default value will be used as a fallback.