Add 'jump relaxing' compile flag to AVR targets (#21566)

ini/avr.ini

 # AVR (8-bit) Common Environment values
 #
 [common_avr8]
+build_flags       = ${common.build_flags} -Wl,--relax
 board_build.f_cpu = 16000000L
 src_filter        = ${common.default_src_filter} +<src/HAL/AVR>