bootstrap: Set CMAKE_SYSTEM_NAME=Darwin on Apple platforms

compiler-rt's CMake setup seems to have special logic for Apple
platforms that works poorly when this is not set.

Author: madsmtm

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

@@ -702,13 +702,20 @@ fn configure_cmake(
             // macOS
             cfg.define("CMAKE_SYSTEM_NAME", "Darwin");
         } else if target.contains("ios") {
-            cfg.define("CMAKE_SYSTEM_NAME", "iOS");
+            // FIXME(madsmtm): compiler-rt's CMake setup is kinda weird, it seems like they do
+            // version testing etc. for macOS (i.e. Darwin), even while building for iOS?
+            //
+            // So for now we set it to "Darwin", but ideally this should be set to "iOS".
+            cfg.define("CMAKE_SYSTEM_NAME", "Darwin");
         } else if target.contains("tvos") {
-            cfg.define("CMAKE_SYSTEM_NAME", "tvOS");
+            // FIXME(madsmtm): See above.
+            cfg.define("CMAKE_SYSTEM_NAME", "Darwin");
         } else if target.contains("visionos") {
-            cfg.define("CMAKE_SYSTEM_NAME", "visionOS");
+            // FIXME(madsmtm): See above.
+            cfg.define("CMAKE_SYSTEM_NAME", "Darwin");
         } else if target.contains("watchos") {
-            cfg.define("CMAKE_SYSTEM_NAME", "watchOS");
+            // FIXME(madsmtm): See above.
+            cfg.define("CMAKE_SYSTEM_NAME", "Darwin");
         } else if target.contains("none") {
             // "none" should be the last branch
             cfg.define("CMAKE_SYSTEM_NAME", "Generic");