riscv: fix use of incorrectly sized variables

The RISC-V arch incorrectly uses 32-bit instead of 64-bit variables in
several places. Fix this.
In addition, BITS_PER_LONG is set to 64 on RV64I systems.

Signed-off-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>