build on aarch64 broken

[ericvh]

Dup2 not available in aarch64 golang.

# github.com/gocircuit/circuit/kit/debug
kit/debug/sigpanic_save_panic_trace.go:28:2: undefined: syscall.Dup2
Makefile:2: recipe for target 'nix' failed
make: *** [nix] Error 2

[hoijui]

This seems related to #14. A related upsteam (golang) issue can be found here: golang/go#11981

so if I understood the upstream comments right, you should be able to replace syscall.Dup2 with syscall.Dup3, on ARM64.

Edit: Dup 3 takes an additional parameter flags int, which I assume could be set to 0 (comments on that?).