[RISCV] Move iterators from sync.md to iterators.md

[RISCV] Move iterators from sync.md to iterators.md

Like the previous two patches this moves the iterators
that are in sync.md to iterators.md.

OK? build and tested for riscv64-linux-gnu.

gcc/ChangeLog:

	* config/riscv/sync.md (any_atomic, atomic_optab): Move to ...
	* config/riscv/iterators.md: Here.