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

Zbb extension support for bit manipulation instructions

configname: CONFIG_RISCV_ISA_ZBB

Linux Kernel Configuration
└─>Platform type
└─>Zbb extension support for bit manipulation instructions
In linux kernel since version 4.14.326 (release Date: 2023-09-23)  
Adds support to dynamically detect the presence of the ZBB
extension (basic bit manipulation) and enable its usage.

The Zbb extension provides instructions to accelerate a number
of bit-specific operations (count bit population, sign extending,
bitrotation, etc).

If you don't know what to do here, say Y.