MinGW: disable self-contained mode when cross compiling

When cross compiling users have to provide own linker and libraries anyway.
Using rust provided MinGW crt objects is harmful here and has no benefits.

Author: mati865

src/librustc_codegen_ssa/back/link.rs

@@ -1295,7 +1295,9 @@ fn crt_objects_fallback(sess: &Session, crate_type: CrateType) -> bool {
         Some(CrtObjectsFallback::Musl) => sess.crt_static(Some(crate_type)),
         // FIXME: Find some heuristic for "native mingw toolchain is available",
         // likely based on `get_crt_libs_path` (https://github.com/rust-lang/rust/pull/67429).
-        Some(CrtObjectsFallback::Mingw) => sess.target.target.target_vendor != "uwp",
+        Some(CrtObjectsFallback::Mingw) => {
+            sess.host == sess.target.target && sess.target.target.target_vendor != "uwp"
+        }
         // FIXME: Figure out cases in which WASM needs to link with a native toolchain.
         Some(CrtObjectsFallback::Wasm) => true,
         None => false,