[Mono] Restore old code to solve the recent SpanHelpers regressions (dotnet#75917)

* bring back the old code...

* bring back more old code

* Use an ifdef around clr code instead of a separate file

* Delete SpanHelpers.Clr.cs

* Remove a remaining INumber<T> helper from mono

Co-authored-by: Jeff Handley <[email protected]>

Author: adamsitnik

src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems

@@ -2462,4 +2462,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\Numerics\IUnaryPlusOperators.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Numerics\IUnsignedNumber.cs" />
   </ItemGroup>
+  <ItemGroup Condition="'$(FeatureMono)' == 'true'">
+    <Compile Include="$(MSBuildThisFileDirectory)System\SpanHelpers.Mono.cs" />
+  </ItemGroup>
 </Project>

src/libraries/System.Private.CoreLib/src/System/MemoryExtensions.cs

@@ -1617,6 +1617,7 @@ ref Unsafe.As<T, byte>(ref MemoryMarshal.GetReference(span)),
                                 Unsafe.Add(ref valueRef, 2),
                                 span.Length);
 
+#if !MONO // We don't have a mono overload for 4 values
                         case 4:
                             return SpanHelpers.LastIndexOfAnyValueType(
                                 ref spanRef,
@@ -1625,6 +1626,7 @@ ref Unsafe.As<T, byte>(ref MemoryMarshal.GetReference(span)),
                                 Unsafe.Add(ref valueRef, 2),
                                 Unsafe.Add(ref valueRef, 3),
                                 span.Length);
+#endif
 
                         default:
                             return LastIndexOfAnyProbabilistic(ref Unsafe.As<short, char>(ref spanRef), span.Length, ref Unsafe.As<short, char>(ref valueRef), values.Length);