Android & emscripten: Make buildscript work on m1 (arm64) macOS

[Ghabry]

Requires a newer NDK version which drops support for older Android SDKs <19.

Though this is no problem for us as we already require API Level 20.

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For context: fdelapena set up a m1 mac. This will replace my awful, slow macOS builder.

The m1 is fast so I consider also offloading the Android build to it. The Android build is especially bad on our slow VPS because it compiles 4 times.

(then the main builder will only build Linux and emscripten)

This is a draft until the migration is done.

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Emscripten was fixed because I finally have a test system available xD