Windows x86: Change `i128` to return via the vector ABI

Clang and GCC both return `i128` in xmm0 on windows-msvc and
windows-gnu. Currently, Rust returns the type on the stack. Add a
calling convention adjustment so we also return scalar `i128`s using the
vector ABI, which makes our `i128` compatible with C.

In the future, Clang may change to return `i128` on the stack for its
`-msvc` targets (more at [1]). If this happens, the change here will
need to be adjusted to only affect MinGW.

Link: https://github.com/rust-lang/rust/issues/134288