CONFIG_ARC_PLAT_FPGA_LEGACY is not available for thedefaultarchitecture x86.
Result is shown for architecture arc

"Legacy" ARC FPGA dev Boards

configname: CONFIG_ARC_PLAT_FPGA_LEGACY

Linux Kernel Configuration
└─>"Legacy" ARC FPGA dev Boards
In linux kernel since version 3.1 (release Date: 2011-10-24)  
Support for ARC development boards, provided by Synopsys.
These are based on FPGA or ISS. e.g.
- ARCAngel4
- ML509
- MetaWare ISS


is selected by
CONFIG_ISS_SMP_EXTN