Fix the AVR assembler so that it will correctly issue warnings about skipped instruct...

Fix the AVR assembler so that it will correctly issue warnings about skipped instructions even if subsections are used.

	PR 21621
	* config/tc-avr.h (struct avr_frag_data): Add prev_opcode field.
	(TC_FRAG_INIT): Define.
	(avr_frag_init): Add prototype.
	* config/tc-avr.c (avr_frag_init): New function.
	(avr_operands): Replace static local 'prev' variable with
	prev_opcode field in current frag.
	* testsuite/gas/avr/pr21621.s: New test source file.
	* testsuite/gas/avr/pr21621.d: New test driver file.
	* testsuite/gas/avr/pr21621.s: New test error output file.