target/105753: Fix ICE in add_clobbers due to extra PARALLEL in insn.

This patch removes the superfluous parallel in [u]divmod patterns in
the AVR backend.  Effect of extra parallel is that add_clobbers reaches
gcc_unreachable() because the clobbers for [u]divmod are missing.
If an insn has multiple parts like clobbers, the parallel around the
parts of the insn pattern is implicit.

gcc/
	PR target/105753
	Backport from 2023-05-20 https://gcc.gnu.org/r14-1016
	* config/avr/avr.md (divmodpsi, udivmodpsi, divmodsi, udivmodsi):
	Remove superfluous "parallel" in insn pattern.
	([u]divmod<mode>4): Tidy code.  Use gcc_unreachable() instead of
	printing error text to assembly.

gcc/testsuite/
	PR target/105753
	Backport from 2023-05-20 https://gcc.gnu.org/r14-1016
	* gcc.target/avr/torture/pr105753.c: New test.