CONFIG_MACH_STM32F746 is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

STMicroelectronics STM32F746

configname: CONFIG_MACH_STM32F746

Linux Kernel Configuration
└─>STMicroelectronics STM32 family
└─>STMicroelectronics STM32F746
In linux kernel since version 3.10 (release Date: 2013-06-30)  

is selected by
CONFIG_STM32F7_DEBUG_UART