CONFIG_MACH_SPEAR600 is not available for thedefaultarchitecture x86.
Result is shown for architecture arm
default or selected kernelversion does not have config value CONFIG_MACH_SPEAR600.
Result is shown for kernelversion 6.9.9

CONFIG_MACH_SPEAR600 (not configurable)

configname: CONFIG_MACH_SPEAR600

Linux Kernel Configuration
└─>ST SPEAr Family
└─>MACH_SPEAR600
In linux kernel since version 2.6.35 (release Date: 2010-08-01)  
Supports ST SPEAr600 boards configured via the device-tree