[portability] Move to non-GCD based SIGINT catching.
[portability] Move to non-GCD based SIGINT catching.

 - This should eventually be moved out into a helper class for orderly signal
   handling, but this works for now to eliminate our last use of GCD.

apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild