Pass system header paths to fallback parser

If a macro needed to be parsed using the fallback parser but the macro
used items included in a builtin header, the parsing would fail since
the location of the header would be unknown.

Author: dimtpap

bindgen-tests/tests/expectations/tests/macro-fallback-include-builtin.rs

@@ -0,0 +1,7 @@
+// bindgen-flags: --clang-macro-fallback --allowlist-item TEST
+// Ensures that the fallback path for macros doesn't silently
+// fail because of builtin headers not being found.
+
+#include <stddef.h>
+
+#define TEST ((size_t)42)

bindgen-tests/tests/headers/macro-fallback-include-builtin.h

@@ -871,7 +871,10 @@ impl Bindings {
                 for path in search_paths {
                     if let Ok(path) = path.into_os_string().into_string() {
                         options.clang_args.push("-isystem".into());
-                        options.clang_args.push(path.into_boxed_str());
+                        options.clang_args.push(path.clone().into_boxed_str());
+
+                        options.fallback_clang_args.push("-isystem".into());
+                        options.fallback_clang_args.push(path.into_boxed_str());
                     }
                 }
             }