Allow bootstrap to work when building a toolchain on Apple Silicon

[edymtt]

Allow bootstrap to work when building a toolchain on Apple Silicon

Motivation:

We are ironing out the current issues we are having when building a toolchain on Apple Silicon, and currently we are hitting the following exception when bootstrap starts (rdar://99040114)

Traceback (most recent call last):
  File "/Volumes/Storage/swift_public/swift-arm64/swiftpm/Utilities/bootstrap", line 825, in <module>
    main()
  File "/Volumes/Storage/swift_public/swift-arm64/swiftpm/Utilities/bootstrap", line 65, in main
    args.func(args)
  File "/Volumes/Storage/swift_public/swift-arm64/swiftpm/Utilities/bootstrap", line 329, in build
    parse_build_args(args)
  File "/Volumes/Storage/swift_public/swift-arm64/swiftpm/Utilities/bootstrap", line 225, in parse_build_args
    args.bootstrap_dir = os.path.join(args.target_dir, "bootstrap")
AttributeError: 'Namespace' object has no attribute 'target_dir'

Modifications:

Generalization of a check in bootstrap

Result:

We should be able to start the CMake builds for swiftpm

[edymtt]

@swift-ci please test

[neonichu]

Looks fine. I wouldn't be surprised if we encounter more follow-on issues, the cross-compilation support of bootstrap is really minimal.

[edymtt]

@swift-ci please smoke test