Tweak build flags for internalDup2 on darwin arm64

[lavoiesl]

Fixes an issue that prevents building on darwin amr64 (M1 macs):

$ go build .
# github.com/linkedin/Burrow/core
core/logger.go:172:2: undefined: internalDup2
core/logger.go:173:2: undefined: internalDup2

My understanding is that open_out_log_unix.go was designed to be used everywhere except windows and linux_arm64, as evidenced by the comment:

linux_arm64 doesn't have syscall.Dup2, so use the nearly identical syscall.Dup3 instead

However, the build flags in open_out_log_unix.go were also excluding darwin_arm64, which was not provided (rightly so) by open_out_log_linux_arm64.go.

I decided to go the route of making the build flags of open_out_log_unix.go only exclude linux_arm64 instead of adding mentions of darwin, because I felt it was more semantic; a better representation of the inherent non-overlapping union of the 3 flags:

1. windows
2. linux_arm64
3. !windows && !linux_arm64 (not the first 2)