Don't define HAS_CUSTOM_BLOCKS on mono

This change was originally made here in dotnet#106764, but was reverted due to performance issues on AOT and interp environments. Since we only use Mono in a JIT environment, we can safely reapply this change.

Issue to track the underlying issue on Mono: dotnet#106822

Author: martinpotter

src/libraries/System.Private.CoreLib/src/System/SpanHelpers.ByteMemOps.cs

@@ -1,7 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
-#if TARGET_AMD64 || TARGET_ARM64 || (TARGET_32BIT && !TARGET_ARM) || TARGET_LOONGARCH64
+#if !MONO && (TARGET_AMD64 || TARGET_ARM64 || (TARGET_32BIT && !TARGET_ARM) || TARGET_LOONGARCH64)
 // JIT is guaranteed to unroll blocks up to 64 bytes in size
 #define HAS_CUSTOM_BLOCKS
 #endif