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

CONFIG_SYS_SUPPORTS_HIGHMEM (not configurable)

configname: CONFIG_SYS_SUPPORTS_HIGHMEM

Linux/MIPS Kernel Configuration
└─>Kernel type
└─>SYS_SUPPORTS_HIGHMEM
In linux kernel since version 2.6.15 (release Date: 2006-01-02)  
is selected by
CONFIG_CPU_MIPS32_R5_XPA

is depended by
CONFIG_NLM_XLP_BOARD