Add rseq syscall to strict sandbox mode

After a recent update, the rseq syscall is required for the strict sandbox mode on rolling release distros like arch.

Restartable sequences are a lightweight interface that allows user-level code to be executed atomically relative to scheduler preemption and signal delivery. Typically used for implementing per-cpu operations.

It allows user-space to perform update operations on per-cpu data without requiring heavy-weight atomic operations.