Being able to extract SDKs from Command Line Tools for Xcode instead
of the full Xcode package saves bandwidth, disk space, and time.
Two scripts are added:
* tools/gen_sdk_package_tools.sh: extracts SDKs from installed
Command Line Tools on macOS (/Library/Developer/CommandLineTools)
or from a path specified via XCODE_TOOLS_DIR
* tools/gen_sdk_package_tools_dmg.sh: unpacks the content of provided
Command_Line_Tools_for_Xcode.dmg and extracts SDKs via the above
script.
Tested with Command Line Tools for Xcode 12.x.
