CONFIG_RISCV_M_MODE is not available for thedefaultarchitecture x86.
Result is shown for architecture riscv

Build a kernel that runs in machine mode

configname: CONFIG_RISCV_M_MODE

Linux Kernel Configuration
└─>Build a kernel that runs in machine mode
In linux kernel since version 4.14.326 (release Date: 2023-09-23)  
Select this option if you want to run the kernel in M-mode,
without the assistance of any other firmware.

depends
NOT CONFIG_MMU