ARM: ffixed option for R12

I would like to reserve R12 for my application in Thumb mode, but I don't know how to make it with Keil uVisionV5. GCC has -ffixed-reg (don't know if or how it works). I am compiling now for Arm Cortex M7. Doe's compiled c code use upper registers R8..R12 at all or when they are used.