Add riscv64 as a target architecture

[lukaszsobala]

Hi,

Thanks for developing FAMSA!

Firstly I must say that I am not a software engineer, I am just using the LLM coding tools. I am proposing these changes as is and I will not be able to answer technical questions about the approach. What I can say is that the changes are enough to port FAMSA to riscv64 (-march=rv64g). The program compiles (gmake PLATFORM=rv64g) and works.

Some simplification of the nested loop in refresh.mk is also proposed.

The specific riscv64 device I possess has the architecture rv64imafdvcsu, which also supports compressed and vector extensions (rv64gcv). It may be possible that they can make the program faster. I do not know which architecture target is most common for riscv64 software. If superflouous, the target rv64gcv can be removed.