Add `poll_darwin_arm.go` for iOS gomobile support

[tptee]

Confirmed that this compiles and runs on iOS by:

• Running gomobile bind -target=ios .
• Linking the resulting framework into a simple iOS project
• Importing the framework and calling its header-exposed initializer.

This is assuming that the kqueue implementation for x86 Darwin is the same as on ARM Darwin.

[tptee]

Weird, tests pass on macOS for me. Anything I'm missing?

[Kubuxu]

For sure buildtag that locks this code to ios.

[tptee]

Oops! Added the build tag and tests pass now 👍

[whyrusleeping]

@Kubuxu @tptee wait a sec, does this only work on iOS? Or does this same fix also work for darwin (aka, normal macbooks)?

[whyrusleeping]

(also, apologies for the great delay)

[whyrusleeping]

ah, nvm. I see that this is just a slight fork of the base poll_bsd.go code.

@Kubuxu is this RFM?