CONFIG_MACH_SPEAR600 is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

SPEAr600

configname: CONFIG_MACH_SPEAR600

Linux Kernel Configuration
└─>SPEAr600
In linux kernel since version 2.6.35 (release Date: 2010-08-01)  
Supports ST SPEAr600 Machine

selects

CONFIG_USE_OF

depends
CONFIG_ARCH_SPEAR6XX |
| CONFIG_PLAT_SPEAR

is depended by
CONFIG_BOARD_SPEAR600_DT