CONFIG_CPU_LOONGSON64 is not available for thedefaultarchitecture x86.
Result is shown for architecture mips

Loongson 64-bit CPU

configname: CONFIG_CPU_LOONGSON64

CPU type
└─>Loongson 64-bit CPU
In linux kernel since version 4.14.326 (release Date: 2023-09-23)  
The Loongson GSx64(GS264/GS464/GS464E/GS464V) series of processor
cores implements the MIPS64R2 instruction set with many extensions,
including most 64-bit Loongson-2 (2H, 2K) and Loongson-3 (3A1000,
3B1000, 3B1500, 3A2000, 3A3000 and 3A4000) processors. However, old
Loongson-2E/2F is not covered here and will be removed in future.