Revert removal of SuppressGCTransition from SystemNative_GetTimestamp…

…() (dotnet#27473)

* Revert removal of SuppressGCTransition from SystemNative_GetTimestamp()

* Insert GC_POLL before statement with unmanaged call.

* JIT test for insertion of GCPoll

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

src/Common/src/CoreLib/Interop/Unix/System.Native/Interop.GetTimestamp.cs

@@ -12,7 +12,7 @@ internal static partial class Sys
         internal static extern ulong GetTimestampResolution();
 
         [DllImport(Libraries.SystemNative, EntryPoint = "SystemNative_GetTimestamp", ExactSpelling = true)]
-        // [SuppressGCTransition] // https://github.com/dotnet/coreclr/issues/27465
+        [SuppressGCTransition]
         internal static extern ulong GetTimestamp();
     }
 }