Add support for riscv64

[yuzibo]

Hi, I noticed the openmsx that has support riscv64 arch, but it seems the openmsx-debugger does not support riscv64. The patch I sent for Debian openmsx-debugger apply here maybe also:

https://bugs.debian.org/cgi-bin/bugreport.cgi?att=1;bug=1010507;filename=fix-riscv64-ftbfs.patch;msg=5

If you need me to do more test on real riscv64 hardware, please let me know:)