AVR: Fix building LibF7 after switching to C23.

Since r15-5327, GNU-C23 is being used as C language default.
libf7.h doesn't assume headers like stdbool.h are present
and defines bool, true and false on its own.

libgcc/config/avr/libf7/
	* libf7.h (bool, true, false): Don't define in C23 or higher.