[compiler-rt] Fix an MSVC build issue. (PR #111557)

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

If the compiler can't handle `.S` files, I'm surprised if this is the only case you're running into - aren't there loads of other files within compiler-rt/lib/builtins? Or perhaps not?

I think the change looks reasonable, but I'm not entirely familiar with the context/scope here.

https://github.com/llvm/llvm-project/pull/111557