The main stack is not defined in the RTOS code, it is part of the normal C runtime because it is already in use upon main() function entry and before chSysInit() is called (you cannot clear a stack you are running on), the allocation is statically done in the linker file.
The fill options for system stacks are available in the startup file crt0xx.S: - CRT0_STACKS_FILL_PATTERN - CRT0_INIT_STACKS