CONFIG_ARM64_RAS_EXTN is not available for thedefaultarchitecture x86.
Result is shown for architecture arm64

Enable support for RAS CPU Extensions

configname: CONFIG_ARM64_RAS_EXTN

Linux Kernel Configuration
└─>Kernel Features
└─>ARMv8.2 architectural features
└─>Enable support for RAS CPU Extensions
In linux kernel since version 4.2 (release Date: 2015-08-30)  
CPUs that support the Reliability, Availability and Serviceability
(RAS) Extensions, part of ARMv8.2 are able to track faults and
errors, classify them and report them to software.

On CPUs with these extensions system software can use additional
barriers to determine if faults are pending and read the
classification from a new set of registers.

Selecting this feature will allow the kernel to use these barriers
and access the new registers if the system supports the extension.
Platform RAS features may additionally depend on firmware support.