AVR: target/87376 - Use nop_general_operand for DImode inputs.

The avr-dimode.md expanders have code like  emit_move_insn(acc_a, operands[1])
where acc_a is a hard register and operands[1] might be a non-generic
address-space memory reference.  Such loads may clobber hard regs since
some of them are implemented as libgcc calls /and/ 64-moves are
expanded as eight byte-moves, so that acc_a or acc_b might be clobbered
by such a load.

This patch simply denies non-generic address-space references by using
nop_general_operand for all avr-dimode.md input predicates.
With the patch, all memory loads that require library calls are issued
before the expander codes from avr-dimode.md are run.

	PR target/87376
gcc/
	* config/avr/avr-dimode.md: Use "nop_general_operand" instead
	of "general_operand" as predicate for all input operands.

gcc/testsuite/
	* gcc.target/avr/torture/pr87376.c: New test.