RISC-V: Implement full-featured iterator for riscv_subset_list [NFC]

RISC-V: Implement full-featured iterator for riscv_subset_list [NFC]

This commit implements a full-featured iterator for the
riscv_subset_list, that it able to use range-based-for-loop.

That could simplfy the code in the future, and make it more readable,
also more compatible with standard C++ containers.

gcc/ChangeLog:

	* config/riscv/riscv-c.cc (riscv_cpu_cpp_builtins): Use
	range-based-for-loop.
	* config/riscv/riscv-subset.h (riscv_subset_list::iterator):
	New.
	(riscv_subset_list::const_iterator): New.