[RISCV] Add the list of operand modifiers to riscv.md too

[RISCV] Add the list of operand modifiers to riscv.md too

To make it easier to find operands modifiers while in the md
file, add the list of modifiers to the top of the md file.
This is similar to i386 target.

OK? Built and tested for riscv32-linux-gnu and riscv64-linux-gnu.

gcc/ChangeLog:

	* config/riscv/riscv.cc (riscv_print_operand): Make a mention to
	keep the list in riscv.md in sync with this list.
	* config/riscv/riscv.md: Add list of modifiers as comments.