Cross-compiling from Ubuntu for aarch64

In the table of supported platforms it looks like the only way to target arm64 architecture for lambda is to compile on Amazon Linux 2.

Would it be possible to support cross-compiling from other OS's? Specifically Ubuntu 22.04?

My lambda function seems to build fine but its an error in the packaging stage:

```
cd .../build/lambda/src/aws && ../../vcpkg_installed/arm64-linux-release/share/aws-lambda-runtime/packager .../build/lambda/src/aws/test_lambda
not a dynamic executable
make[3]: *** [src/aws/CMakeFiles/aws-lambda-package-test_lambda.dir/build.make:73: src/aws/CMakeFiles/aws-lambda-package-test_lambda] Error 1
make[3]: Leaving directory '.../build/lambda'
make[2]: *** [CMakeFiles/Makefile2:3032: src/aws/CMakeFiles/aws-lambda-package-test_lambda.dir/all] Error 2
make[2]: Leaving directory '.../build/lambda'
make[1]: *** [CMakeFiles/Makefile2:3039: src/aws/CMakeFiles/aws-lambda-package-test_lambda.dir/rule] Error 2
make[1]: Leaving directory '..../build/lambda'
make: *** [Makefile:213: src/aws/CMakeFiles/aws-lambda-package-test_lambda.dir/rule] Error 2
```

Start with the supported-platforms table and the aws-lambda-runtime/packager invocation shown in the report, then reproduce the CMake packaging target on Ubuntu 22.04 for arm64. Done means the test_lambda target packages successfully from Ubuntu without the “not a dynamic executable” failure and the supported-platform documentation reflects the result.

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