Swift should support kCFStringEncodingDOSLatinUS on Linux

[microtherion]

This might be another issue classified under the #1015 umbrella.

Summary:
The String(data:encoding:) constructor fails on Linux when encoding is kCFStringEncodingDOSLatinUS.

Steps To Reproduce:
ZIPFoundation is doing the following:

        let dosLatinUS = 0x400
        let dosLatinUSEncoding = CFStringEncoding(dosLatinUS)
        let dosLatinUSStringEncoding = CFStringConvertEncodingToNSStringEncoding(dosLatinUSEncoding)
        let codepage437 = String.Encoding(rawValue: dosLatinUSStringEncoding)
        let encoding = self.centralDirectoryStructure.usesUTF8PathEncoding ? .utf8 : codepage437

This code works on Apple platforms, but fails on Linux.

The reason CP437 is needed for ZIP file decoding is that the ZIP Specification specifies that the encoding is CP437, unless a UTF-8 flag is set. So for legacy ZIP files, CP437 must be assumed (although in practice, encodings were far more random).

[YOCKOW]

Thank you. Added this to #1015.
However, I want to note that kCFStringEncodingDOSLatinUS is not one of CFStringBuiltInEncodings but one of External String Encodings that is not exposed to String.Encoding (yet).

In the context of the pitch about string encoding names, a suggestion to add new static properties to String.Encoding received some kind of negative feedbacks.