Microblaze: Fixed missing save of r18 in fast_interrupt. Register 18 is used as a clobber register, and must be stored when entering a fast_interrupt. Before this fix, register 18 was only saved if it was used directly in the interrupt function.

    However, if the fast_interrupt function called a function that used
    r18, the register would not be saved, and thus be mangled
    upon returning from the interrupt.

	* config/microblaze/microblaze.c (microblaze_must_save_register): Check
	for fast_interrupt.