CONFIG_SPARC64 is not available for thedefaultarchitecture x86.
Result is shown for architecture sparc64

CONFIG_SPARC64 (not configurable)

configname: CONFIG_SPARC64

Linux/UltraSPARC Kernel Configuration
└─>SPARC64
In linux kernel since version 2.6.12  
SPARC is a family of RISC microprocessors designed and marketed by
Sun Microsystems, incorporated. This port covers the newer 64-bit
UltraSPARC. The UltraLinux project maintains both the SPARC32 and
SPARC64 ports; its web page is available at
<http://www.ultralinux.org/>.