Workaround memset alignment sensitivity (dotnet/coreclr#24302)

* Workaround memset alignment sensitivity

memset is up to 2x slower on misaligned block on some types of hardware. The problem is uneven performance of "rep stosb"
used to implement the memset in some cases. The exact matrix on when it is slower and by how much is very complex.

This change workarounds the issue by aligning the memory block before it is passed to memset and filling in the potential misaligned
part manually. This workaround will regress performance by a few percent (<10%) in some cases, but we will gain up to 2x improvement
in other cases.

Fixes dotnet#24300

Signed-off-by: dotnet-bot <dotnet-bot@microsoft.com>

Author: jkotas

src/Common/src/CoreLib/System/SpanHelpers.cs

@@ -24,7 +24,9 @@ public static unsafe void ClearWithoutReferences(ref byte b, nuint byteLength)
                 return;
 
 #if CORECLR && (AMD64 || ARM64)
-            if (byteLength > 4096)
+            // The exact matrix on when RhZeroMemory is faster than InitBlockUnaligned is very complex. The factors to consider include
+            // type of hardware and memory aligment. This threshold was chosen as a good balance accross different configurations.
+            if (byteLength > 768)
                 goto PInvoke;
             Unsafe.InitBlockUnaligned(ref b, 0, (uint)byteLength);
             return;