xtensa: bfd: add special case to loop alignment check

xtensa: bfd: add special case to loop alignment check

check_loop_aligned is used during link time relaxation to only allow
transformations that don't violate loop body alignment requirements.
Assembler can relax loops that have too long body by adding instructions
between the loop instruction and the loop body. check_loop_aligned must
check alignment of the first instruction of the actual loop body.
Detect loop / rsr.lend / wsr.lbeg sequence used in assembly time
relaxation and adjust alignment check when it's detected.

bfd/
2019-08-01  Max Filippov  <jcmvbkbc@gmail.com>

	* elf32-xtensa.c (insn_num_slots, get_rsr_lend_opcode)
	(get_wsr_lbeg_opcode): New functions.
	(check_loop_aligned): Detect relaxed loops and adjust loop_len
	and insn_len for the first actual instruction of the loop.