Runtime: restrict behaviour to cygwin

[compnerd]

What's in this pull request?

Resolved bug number: (SR-)

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__int128 is not available as an extension on MSVC nor clang for Windows.
Restrict the behaviour to cygwin only.

[gribozavr]

@swift-ci Please test and merge

[tinysun212]

@compnerd, @gribozavr, I'm sorry so late comment.
clang with -fms-extension (without -fms-compatibility) can recognize the __int128, and that worked well for 64bit MSVC.
I think this will work for 32bit MSVC, but will not return two word type for 64bit MSVC. (this means two uintptr_t type not DWORD). How about change this part work for both 32/64bit MSVC using #if/#endif?

[compnerd]

Well, cl nor clang-cl seem to recognize __int128 when I tested it (on x86, x86_64, nor ARM). Does it actually work with cl?

[tinysun212]

@compnerd, As you know, the driver clang-cl invokes compiler including flags -fms-extension, -fms-compatibility, and -fms-compatibility-version=xx. clang-cl does not recognize __int128.

I used clang with option -fms-extension and -fms-compatibility-version=19 without -fms-compatibility. The clang with the options could recognize __int128 and compile MSVC2015 (at least for Update 2) library headers with some more options.

This approach needed lesser patch to Swift source and could generate some Swift stdlib for 64bit MSVC.