Issue Description:

For example, with following code, users expected to create a new file (if it doesn’t exist) with permission that respects current umask.

import Foundation
try! Data().write(to: URL(fileURLWithPath: "file"))

On macOS, In case umask is 022, it creates file with permission 0644 on macOS.
in case umask is 0, it creates file with permission 0666.
Therefore, default mode to open(2) seems having 0666.

However, on Linux, since corelibs Foundation is using default mode 0600 to open(2) (called via _CFOpenFileWithMode()), in case umask is 022, it creates file with permission 0600 instead of 0644.
https://github.com/apple/swift-corelibs-foundation/blame/master/Sources/Foundation/NSData.swift#L438

This behavior's inconsistency surprising Foundation library users.