This removes the compiler_rt.setXmm0 hack. Instead, for
the functions that use i128 or u128 in their parameter and
return types, we use `@Vector(2, u64)` which generates
the LLVM IR `<2 x i64>` type that matches what Clang
generates for `typedef int ti_int __attribute__ ((mode (TI)))`
when targeting Windows x86_64.
It's needed because LLVM emits library calls to compiler-rt when hardware lacks
functionality, for example, 64-bit integer multiplication on 32-bit x86.
This library is automatically built as-needed for the compilation target and
then statically linked and therefore is a transparent dependency for the
programmer.
Any bugs should be solved by trying to duplicate the bug upstream.
If the bug exists upstream, get it fixed with the LLVM team and then port
the fix downstream to Zig.
If the bug only exists in Zig, something went wrong porting the code,
and you can run the C code and Zig code side by side in a debugger
to figure out what's happening differently.
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