try initial riscv64 support

Hi,

As this issue concerned, I tried to add riscv64 support to ltrace. Now it worked a bit. I tested it on qemu-system-riscv64 with linux kernel 5.10. ltrace ls got similar result with x86_64, all 433 testcases there passed 381.

But I do leave some questions in the code and there must still other questions or mistakes somewhere I don't know. Please review and help it out. !4 (merged)