RISC-V: Add new constraint R for register even-odd pairs

Although this constraint is not currently used for any instructions, it is very
useful for custom instructions. Additionally, some new standard extensions
(not yet upstream), such as `Zilsd` and `Zclsd`, are potential users of this
constraint. Therefore, I believe there is sufficient justification to add it
now.

gcc/ChangeLog:

	* config/riscv/constraints.md (R): New constraint.
	* doc/md.texi: Document new constraint `R`.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/constraint-R.c: New.