CONFIG_MIPS32_COMPAT is not available for thedefaultarchitecture x86.
Result is shown for architecture 64BIT or mips

Kernel support for Linux/MIPS 32-bit binary compatibility

modulename: linux32.ko

configname: CONFIG_MIPS32_COMPAT

Linux Kernel Configuration
└─>Executable file formats
└─>Kernel support for Linux/MIPS 32-bit binary compatibility
In linux kernel since version 2.6.12  
Select this option if you want Linux/MIPS 32-bit binary
compatibility. Since all software available for Linux/MIPS is
currently 32-bit you should say Y here.

source code:
depends
CONFIG_64BIT