Fix for Foundation on Android 6.0

[Molanda]

Foundation's Host.swift uses getifaddrs and freeifaddrs, which currently causes Android 6.0 or below to fail to link.

This PR changes these functions so they are called using dlsym, which allows Android 6.0 to link while providing full support on later versions of Android.

This is a replacement for PR #2439.

[compnerd]

Does this not re-open the library each call and then lookup the symbol? That isn't the cheapest operation. Why not make the function a lazy static that will cache the value avoiding the dlopen/dlsym on subsequent calls?

[Molanda]

No, RTLD_NOLOAD returns the shared instance of the library.

getifaddrs and freeifaddrs are only called once, the resulting data is cached, and _resolved is set to true. So there is no benefit to caching the function pointers in this case.

[compnerd]

Ah, good point about the NOLOAD. I’m still worried that future changes may end up calling the function multiple times, looking up the symbol each time.

[Molanda]

Understood. I updated the comment to capture your concern for any future changes.

[compnerd]

@swift-ci please test

[compnerd]

CC: @drodriguez

[drodriguez]

I don't know why CI is not triggering. Someone else might need to trigger it.

[spevans]

@swift-ci please test

[Molanda]

Looks like the Linux build failed for an unrelated reason...

/home/buildnode/jenkins/workspace/swift-corelibs-foundation-PR-Linux/swift-corelibs-libdispatch/src/swift/Block.swift:13:8: error: no such module 'CDispatch'

[spevans]

@swift-ci please test linux