Sandbox: Permit the clone3 system call

Nick Mathewsonrequested to merge
nickm/tor:bug40590_035 into maint-0.3.5

Apparently glibc-2.34 uses clone3, when previously it just used clone.

This MR is against 0.3.5; I am agnostic about how far we should backport it.

Closes #40590 (closed).

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