Missing `--target` triple validation on `make-linux-sdk` command

[xtremekforever]

I found that it's totally possible to create an invalid Swift SDK by passing an incomplete triple to --target, like this:

swift run swift-sdk-generator make-linux-sdk --target aarch64

This generates a swift-sdk.json file with an invalid triple:

{
  "schemaVersion" : "4.0",
  "targetTriples" : {
    "aarch64" : {
      "sdkRootPath" : "ubuntu-jammy.sdk",
      "swiftResourcesPath" : "ubuntu-jammy.sdk/usr/lib/swift",
      "swiftStaticResourcesPath" : "ubuntu-jammy.sdk/usr/lib/swift_static",
      "toolsetPaths" : [
        "toolset.json"
      ]
    }
  }
}

No warning is thrown and it takes the invalid triple like a champ. This Swift SDK fails to cross-compile properly as the compiler doesn't quite know what platform we're trying to use. At a minimum the triple should contain:

• arch: aarch64
• platform: linux- this should be enforced for make-linux-sdk.
• abi: gnu, gnueabihf, etc

We should validate the triple that is passed to ensure we have the right fields, and throw an error if an incomplete triple is passed at the CLI.