Setup Swift SDK builds for Android

[tristanlabelle]

We need to be able to build the Swift SDK (runtime, dispatch, foundation, xctest) for Android as a first step to supporting this platform.

Breakdown:

• [x] repair android and termux confusion
• [x] fix up android SDK selection
• [x] repair PIC code compilation when cross-compiling Windows -> Android
• [x] adjust SDK overlay for NDK changes
• [x] add a module map for Android NDK, allowing using C++ interop and Android NDK's libc++ from Swift: https://github.com/apple/swift/pull/72161/files
• [x] Support or workaround ICU cross-compilation
• [x] Update build.ps1 to support building the SDK for Android targets
  • [x] wire up platform information for individual builds
  • [x] enable NDK selection
  • [x] setup build environment
• [x] Create a GitHub Actions workflow to build Android SDKs: https://github.com/compnerd/swift-build/pull/740
• [x] enable support for building compiler-rt builtins for Android: https://github.com/swiftlang/swift/pull/74754
• [x] Swift C++ interop: fix cross-ref serialization ref: https://github.com/apple/swift/pull/72643

[hyp]

@compnerd I think we're done. Let's close?