CONFIG_AS_HAS_ARMV8_5 is not available for thedefaultarchitecture x86.
Result is shown for architecture arm64

CONFIG_AS_HAS_ARMV8_5 (not configurable)

configname: CONFIG_AS_HAS_ARMV8_5

Linux Kernel Configuration
└─>Kernel Features
└─>ARMv8.5 architectural features
└─>AS_HAS_ARMV8_5
In linux kernel since version 5.2 (release Date: 2019-07-07)  
is selected by
CONFIG_ARM64_MTE