bootstrap: Always set CMAKE_SYSTEM_NAME when cross-compiling

To avoid a panic in cmake-rs that was introduced in:
rust-lang/cmake-rs#158

Author: madsmtm

src/bootstrap/src/core/build_steps/llvm.rs

@@ -645,10 +645,17 @@ fn configure_cmake(
     if !builder.is_builder_target(target) {
         cfg.define("CMAKE_CROSSCOMPILING", "True");
 
+        // NOTE: Ideally, we wouldn't have to do this, and `cmake-rs` would just handle it for us.
+        // But it currently determines this based on the `CARGO_CFG_TARGET_OS` environment variable,
+        // which isn't set when compiling outside `build.rs` (like bootstrap is).
+        //
+        // So for now, we define `CMAKE_SYSTEM_NAME` ourselves, to panicking in `cmake-rs`.
         if target.contains("netbsd") {
             cfg.define("CMAKE_SYSTEM_NAME", "NetBSD");
         } else if target.contains("dragonfly") {
             cfg.define("CMAKE_SYSTEM_NAME", "DragonFly");
+        } else if target.contains("openbsd") {
+            cfg.define("CMAKE_SYSTEM_NAME", "OpenBSD");
         } else if target.contains("freebsd") {
             cfg.define("CMAKE_SYSTEM_NAME", "FreeBSD");
         } else if target.is_windows() {
@@ -659,10 +666,27 @@ fn configure_cmake(
             cfg.define("CMAKE_SYSTEM_NAME", "SunOS");
         } else if target.contains("linux") {
             cfg.define("CMAKE_SYSTEM_NAME", "Linux");
+        } else if target.contains("darwin") {
+            // macOS
+            cfg.define("CMAKE_SYSTEM_NAME", "Darwin");
+        } else if target.contains("ios") {
+            cfg.define("CMAKE_SYSTEM_NAME", "iOS");
+        } else if target.contains("tvos") {
+            cfg.define("CMAKE_SYSTEM_NAME", "tvOS");
+        } else if target.contains("visionos") {
+            cfg.define("CMAKE_SYSTEM_NAME", "visionOS");
+        } else if target.contains("watchos") {
+            cfg.define("CMAKE_SYSTEM_NAME", "watchOS");
+        } else if target.contains("none") {
+            // "none" should be the last branch
+            cfg.define("CMAKE_SYSTEM_NAME", "Generic");
         } else {
             builder.info(&format!(
                 "could not determine CMAKE_SYSTEM_NAME from the target `{target}`, build may fail",
             ));
+            // Fallback, set `CMAKE_SYSTEM_NAME` anyhow to avoid the logic `cmake-rs` tries, and
+            // to avoid CMAKE_SYSTEM_NAME being inferred from the host.
+            cfg.define("CMAKE_SYSTEM_NAME", "Generic");
         }
 
         // When cross-compiling we should also set CMAKE_SYSTEM_VERSION, but in