undo @convention(c) hacks in System.swift (#1412)
undo @convention(c) hacks in System.swift (#1412)

Co-authored-by: Johannes Weiss <d63540ace140105639c46173690300ccf8e978b5@apple.com>

apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio