CONFIG_ARCH_STM32 is not available for thedefaultarchitecture x86.
Result is shown for architecture arm64

STMicroelectronics STM32 SoC Family

configname: CONFIG_ARCH_STM32

Linux Kernel Configuration
└─>Platform selection
└─>STMicroelectronics STM32 SoC Family
In linux kernel since version 3.10 (release Date: 2013-06-30)  
This enables support for ARMv8 based STMicroelectronics
STM32 family, including:
- STM32MP25:
- STM32MP251, STM32MP253, STM32MP255 and STM32MP257.