Use UTF-8 for file path in ObjWriter (#66974)

LLVM already assumes the `char*` it receives is UTF-8. Fixes #66913.

src/coreclr/tools/aot/ILCompiler.Compiler/Compiler/DependencyAnalysis/ObjectWriter.cs

@@ -80,7 +80,7 @@ public class ObjectWriter : IDisposable, ITypesDebugInfoWriter
 #endif
 
         [DllImport(NativeObjectWriterFileName)]
-        private static extern IntPtr InitObjWriter(string objectFilePath, string triple = null);
+        private static extern IntPtr InitObjWriter([MarshalAs(UnmanagedType.LPUTF8Str)] string objectFilePath, string triple = null);
 
         [DllImport(NativeObjectWriterFileName)]
         private static extern void FinishObjWriter(IntPtr objWriter);