sandbox failure on arm64

With legacy/trac#24424 (moved) fixed, Tor builds but it still does not run:

$ ./src/or/tor Sandbox 1
Nov 28 08:35:29.521 [notice] Tor 0.3.2.5-alpha (git-d499a5a708f7298b) running on Linux with Libevent 2.1.8-stable, OpenSSL 1.1.0g, Zlib 1.2.8, Liblzma 5.2.2, and Libzstd 1.3.2.
Nov 28 08:35:29.521 [notice] Tor can't help you if you use it wrong! Learn how to be safe at https://www.torproject.org/download/download#warning
Nov 28 08:35:29.521 [notice] This version is not a stable Tor release. Expect more bugs than usual.
Nov 28 08:35:29.521 [notice] Configuration file "/usr/local/etc/tor/torrc" not present, using reasonable defaults.
Nov 28 08:35:29.525 [notice] Scheduler type KIST has been enabled.
Nov 28 08:35:29.525 [notice] Opening Socks listener on 127.0.0.1:9050
Nov 28 08:35:30.000 [notice] Bootstrapped 0%: Starting

============================================================ T= 1511858130
(Sandbox) Caught a bad syscall attempt (syscall unlinkat)
./src/or/tor(+0x1aa4ac)[0xaaaaccfd54ac]
linux-vdso.so.1(__kernel_rt_sigreturn+0x0)[0xffff8806e6c0]
/lib/aarch64-linux-gnu/libc.so.6(unlink+0x14)[0xffff87a4953c]

under strace:

$ strace -f ./src/or/tor DisableDebuggerAttachment 0 Sandbox 1
[...]
getpid()                                = 25468
getpid()                                = 25468
write(1, "Nov 28 08:36:05.000 [notice] Boo"..., 55Nov 28 08:36:05.000 [notice] Bootstrapped 0%: Starting
) = 55
unlinkat(AT_FDCWD, "/home/weasel/.tor/key-pinning-entries", 0) = -1 ENETDOWN (Network is down)
--- SIGSYS {si_signo=SIGSYS, si_code=SYS_SECCOMP, si_call_addr=0xffffa45d253c, si_syscall=__NR_unlinkat, si_arch=AUDIT_ARCH_AARCH64} ---
write(1, "\n==============================="..., 64
============================================================ T=) = 64
write(1, " 1511858165", 11 1511858165)             = 11
write(1, "\n", 1
)                       = 1
write(1, "(Sandbox) Caught a bad syscall a"..., 48(Sandbox) Caught a bad syscall attempt (syscall ) = 48
[..]