Add a few `#if`s to fix errors when targeting `net461`

src/BizHawk.BizInvoke/BizInvokeUtilities.cs

@@ -82,7 +82,12 @@ namespace BizHawk.BizInvoke
 		private static CustomAttributeBuilder GetAttributeBuilder(object o)
 		{
 			// anything more clever we can do here?
+#if NETSTANDARD2_1_OR_GREATER || NET471_OR_GREATER || NETCOREAPP2_0_OR_GREATER
 			if (o is OutAttribute or InAttribute or IsReadOnlyAttribute)
+#else
+			if (o is OutAttribute or InAttribute
+				|| o.GetType().FullName == "System.Runtime.CompilerServices.IsReadOnlyAttribute") // I think without this, you'd still only hit the below runtime assertion when this project targets e.g. netstandard2.0 but a core targets e.g. net8.0, so unlikely --yoshi
+#endif
 			{
 				return new(o.GetType().GetConstructor(Type.EmptyTypes)!, Array.Empty<object>());
 			}

src/BizHawk.Client.EmuHawk/RetroAchievements/LibRCheevos.cs

@@ -19,7 +19,12 @@ namespace BizHawk.Client.EmuHawk
 	{
 		private const CallingConvention cc = CallingConvention.Cdecl;
 
-		private const UnmanagedType STR_MARSHAL_HINT = UnmanagedType.LPUTF8Str;
+		private const UnmanagedType STR_MARSHAL_HINT
+#if NETSTANDARD2_1_OR_GREATER || NET47_OR_GREATER || NETCOREAPP1_1_OR_GREATER
+			= UnmanagedType.LPUTF8Str;
+#else
+			= UnmanagedType.LPStr; // presumably this will produce mojibake for non-ASCII text but is otherwise safe? CPP confirmed it was the right one so I'm trusting him --yoshi
+#endif
 
 		public enum rc_error_t : int
 		{

src/BizHawk.Client.EmuHawk/debug/DebugMenuItems.cs

@@ -28,7 +28,12 @@ namespace BizHawk.Client.EmuHawk.ForDebugging
 
 		public DebugVSystemMenuItem(string sysID, params string[] extraSysIDs)
 		{
-			SysIDs = new[] { sysID }.Concat(extraSysIDs).ToHashSet();
+			SysIDs = new[] { sysID }.Concat(extraSysIDs)
+#if NET472_OR_GREATER || NETCOREAPP2_0_OR_GREATER
+				.ToHashSet();
+#else // Meziantou.Polyfill covers `IEnumerable<>.ToHashSet` but we're not using that at time of writing --yoshi
+				.ToArray();
+#endif
 			Text = sysID;
 		}
 	}