riscv: Sanitize syscall table indexing under speculation

In the Linux kernel, the following vulnerability has been resolved:

riscv: Sanitize syscall table indexing under speculation

The syscall number is a user-controlled value used to index into the syscall table. Use array_index_nospec() to clamp this value after the bounds check to prevent speculative out-of-bounds access and subsequent data leakage via cache side channels.