Put mach_error_string and mach_task_self back
Put mach_error_string and mach_task_self back

Signed-off-by: Daniel A. Steffen <bc823475b60dbcbd7be1bbdfe095b5f1939d5bd2@apple.com>

frankeh/swift-corelibs-libdispatch,frankeh/swift-corelibs-libdispatch,frankeh/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,frankeh/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch