Optimize set_syscall_retval for riscv

[trasz]

Summary

Optimize set_syscall_retval for riscv by predicting the return
value to be zero.

[mhorne]

LGTM

[kp]

Do we see measurable performance improvements with this?

(Looking at the generated code it does seem to be better, but measurements would make me feel better.)

[trasz]

I wouldn’t expect any visible results. The syscall path overhead is still huge, and there’s not a single reason for it - it’s a “death by thousand cuts” situation, which needs to be resolved in small chunks, like this one.