riscv: Add unused attribute to some registers.hh constants

riscv: Add unused attribute to some registers.hh constants

Three of the constants defined in arch/riscv/registers.hh
(ReturnValueReg, SyscallNumReg, and SyscallPseudoReturnReg) may cause
the compiler to warn that they are unused, which results in an error.
This patch adds M5_VAR_USED attributes to them to stop this.

Change-Id: Ie6389a55e8ffb3d003a47d02e76bdf9fb5219457
Reviewed-on: https://gem5-review.googlesource.com/4040
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Alec Roelke <ar4jc@virginia.edu>