CI should test cross-compilation correctly

Right now GitHub Actions do not do what they should do. Instead of testing for specified targets, they build tests for the host OS and run them there. When proper targets are used, CI fails to compile the tests for various reasons.

- Android targets need to get Rust code linked with Android linker from Android NDK, not system linker.
- Non-native Linux targets need gcc/clang cross-compilers installed for BoringSSL to compile.
- MinGW target for Windows needs either a cross-compiler on Linux, or some more work on Windows.