[stdlib] Disable 128-bit div/mod on 64-bit Windows

[troughton]

64-bit Windows doesn't support the 128-bit operations udivti3, umodti3, divti3, or modti3, which LLVM emits upon encountering this function. Disable dividingFullWidth on Win64 as a temporary fix.

Longer term, Compiler-RT should be amended to support 128-bit operations on Windows (since it's currently gated on being LP64 while Windows uses LLP64). We can then link against the Compiler-RT runtime support library.

[gparker42]

Is there a bug report against compiler-rt for this? We should have a FIXME comment referring to that bug.

The other fix would be to implement those functions ourselves locally. IIRC the arm64 build did something similar for a while.

[troughton]

I'm not aware of any bug report currently – the reason I've not submitted one is I'm not actually sure there is a bug. LLVM emits these instructions, but Windows technically doesn't support them; they wouldn't be found in the MSVC runtime, for instance, and 128-bit integers are only supported as a SSE2 SIMD type (as I understand it).

As for implementing the functions locally: I looked at the Compiler-RT source and found that we'd need to copy across a 250+ line function (https://github.com/apple/swift-compiler-rt/blob/70a977cee7fa82acfd99f2e40b2070cb469d99ed/lib/builtins/udivmodti4.c) and need to define a few custom types to match. That's certainly doable, but seemed a more extreme addition than this PR.

In other words, I'm open to hear the best approach; this is just a stopgap solution to fix the build.