riscv: Add X register names to gpr-nums

When encoding instructions it's sometimes necessary to set a
register field to a precise number. This is easiest to do using
the x<num> naming.

Signed-off-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Anup Patel <anup@brainfault.org>