CONFIG_ARM64 is not available for thedefaultarchitecture x86.
Result is shown for architecture arm64

CONFIG_ARM64 (not configurable)

configname: CONFIG_ARM64

Linux Kernel Configuration
└─>ARM64
In linux kernel since version 3.1 (release Date: 2011-10-24)  
ARM 64-bit (AArch64) Linux support.

selects
CONFIG_ARCH_HAS_ATOMIC64_DEC_IF_POSITIVE
CONFIG_ARCH_HAS_OPP
CONFIG_ARCH_HAS_TICK_BROADCAST
CONFIG_ARCH_USE_CMPXCHG_LOCKREF
CONFIG_ARCH_SUPPORTS_ATOMIC_RMW
CONFIG_ARCH_WANT_OPTIONAL_GPIOLIB
CONFIG_ARCH_WANT_COMPAT_IPC_PARSE_VERSION
CONFIG_ARCH_WANT_FRAME_POINTERS
CONFIG_ARM_AMBA
CONFIG_ARM_ARCH_TIMER
CONFIG_ARM_GIC
CONFIG_BUILDTIME_EXTABLE_SORT
CONFIG_CLONE_BACKWARDS
CONFIG_COMMON_CLK
CONFIG_CPU_PM
CONFIG_DCACHE_WORD_ACCESS
CONFIG_GENERIC_CLOCKEVENTS
CONFIG_GENERIC_CLOCKEVENTS_BROADCAST
CONFIG_GENERIC_CPU_AUTOPROBE
CONFIG_GENERIC_EARLY_IOREMAP
CONFIG_GENERIC_IOMAP
CONFIG_GENERIC_IRQ_PROBE
CONFIG_GENERIC_IRQ_SHOW
CONFIG_GENERIC_SCHED_CLOCK
CONFIG_GENERIC_SMP_IDLE_THREAD
CONFIG_GENERIC_STRNCPY_FROM_USER
CONFIG_GENERIC_STRNLEN_USER
CONFIG_GENERIC_TIME_VSYSCALL
CONFIG_HARDIRQS_SW_RESEND
CONFIG_HAVE_ARCH_JUMP_LABEL
CONFIG_HAVE_ARCH_KGDB
CONFIG_HAVE_ARCH_TRACEHOOK
CONFIG_HAVE_C_RECORDMCOUNT
CONFIG_HAVE_DEBUG_BUGVERBOSE
CONFIG_HAVE_DEBUG_KMEMLEAK
CONFIG_HAVE_DMA_API_DEBUG
CONFIG_HAVE_DMA_ATTRS
CONFIG_HAVE_DMA_CONTIGUOUS
CONFIG_HAVE_DYNAMIC_FTRACE
CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
CONFIG_HAVE_FTRACE_MCOUNT_RECORD
CONFIG_HAVE_FUNCTION_TRACER
CONFIG_HAVE_FUNCTION_GRAPH_TRACER
CONFIG_HAVE_GENERIC_DMA_COHERENT
CONFIG_HAVE_HW_BREAKPOINT
CONFIG_HAVE_MEMBLOCK
CONFIG_HAVE_PATA_PLATFORM
CONFIG_HAVE_PERF_EVENTS
CONFIG_HAVE_PERF_REGS
CONFIG_HAVE_PERF_USER_STACK_DUMP
CONFIG_HAVE_SYSCALL_TRACEPOINTS
CONFIG_IRQ_DOMAIN
CONFIG_MODULES_USE_ELF_RELA
CONFIG_NO_BOOTMEM
CONFIG_OF
CONFIG_OF_EARLY_FLATTREE
CONFIG_OF_RESERVED_MEM
CONFIG_PERF_USE_VMALLOC
CONFIG_POWER_RESET
CONFIG_POWER_SUPPLY
CONFIG_RTC_LIB
CONFIG_SPARSE_IRQ
CONFIG_SYSCTL_EXCEPTION_TRACE