[cmake] Pass /winsysroot explicitly

We were relying on HandleLLVMOptions adding this flag, but that's not
included in compiler-rt. Add the flag explicitly ourselves to ensure
it's present; the duplication of the flag in the parts of the build
which do include HandleLLVMOptions doesn't cause any issues.

Reviewed By: ychen

Differential Revision: https://reviews.llvm.org/D121033